I don't really consider myself a professional (or even above-average) player, I've been at it for a few weeks so far, and a lot of on-and-off in DotA before that. So I figure I'll write a guide on a hero I don't see many people use in solo queue ('sides me, anyway)
Blitzcrank the Great Steam Golem is a bit versatile. He can be tanky if you focus on health/mana thanks to his passive, he can be DPS-y thanks to Overdrive and Power Fist, and, of course, rocket grab makes him especially adept at snagging people who either try to run away or those that are annoying ranged carries.
I tend to have the best success playing him as a sort of passive DPS type, myself, hopefully the guide'll reflect that
I generally spec for cooldown reduction. Your mileage may vary.
Your mileage will definitely vary. Teleport's a given, Blitzcrank needs all the mobility he can get.
But for a second spell, there's a couple of options.
Heal: Health recovery is always useful, however the amount healed is somewhat lackluster late game unless you're stacking heals (and that's been nerfed)
Clairvoyance: Nifty, lets you peer into the bushes to jack that hero waiting to ambush you, or set up a hook from the forest.
Exhaust: Coupled with Rocket Grab, you're pretty likely to kill somebody early game, moreso if you have a nuke-happy teammate nearby.
Cleanse: A free 'Get out of terrible situation' card, depending on where and how you use it.
Clarity: Mana recovery lets you use Rocket Grab more, however by the end of the game you'll have more mana than you'll ever actually use.
Ghost: Good for escaping/chasing before Overdrive is ready.
Ignite: Can help get that First Blood for you in tandem with rocket grab/power fist, but...the damage is rather bad on anything that's not squishy.
Promote: I haven't used it, but it's supposedly quite good for pushing.
Fortify: Team utility, basically.
Rally: Heals for more than Heal (over time), and buffs damage. However the cooldown is long and the ward is stationary, which somewhat limits use.
Smite: I don't use it much, good for farming, I guess.
Flash: There's a few good tricks to do with Flash, they require better reaction time than I possess.
Mana Shield (passive): Blitzcrank's passive is rather nifty. Whenever you get to 20% health it takes half of your current mana and creates a shield. It won't block status effects or spells, but it can (and usually will) mean the difference between living and dying in a bad situation. It returns any unused mana after 10 seconds (though usually I'd head back to base by then to heal)
Granted, early game you'll be out of mana more often than not, and it can only trigger once a minute. So don't be overconfident.
Rocket Grab: Blitzcrank fires his hand at the target, grabbing the first enemy it hits and dragging it to him, dealing 60 damage per skill level. 140 mana cost.
This is a pretty critical skill, and can make the difference between someone running away (or just standing around minding their own business), and giving you a kill. On the downside, it's really mana hungry early game, so try not to miss.
Overdrive: Blitzcrank charges himself up, gaining attack and movespeed for a few seconds before becoming slowed for a short period of time.
Your mileage may vary with this skill, It's definitely a boon when it comes to pushing or backhanding a hero, however you need to keep the slow in mind as well if you screw up and need to make an escape.
Power Fist: Makes Blitzcrank hit like a train (double damage) for his next attack and pops the enemy up into the air.
Quite versatile, as far as damage dealing goes. You farm with it, you ruin an opponent's day with it, etc. However, levelling it does nothing for you but decrease cooldown (which, while extremely good, can be safely put off for a bit). Also works on towers.
Static Field: Blitzcrank's ultimate. Passive hits a random nearby target with a lightning bolt for 'eh' damage, active sprays lightning around in a modest AoE for decent damage.
Underwhelming, but it works for what you use it for. A nice large amount of damage on a hero that survives grab > fist combination. Also rather good for farming.
I tend to go with this order:
1- Rocket Grab
2- Power Fist
3- Rocket Grab
5- Rocket Grab
6- Static Field
Then max Rocket Grab/Overdrive while taking Static Field when available. Power fist comes last (mostly because even with it's long cooldown at level 1, it's available when you need it)
Pay attention to enemy team composition, Snares/Stuns are your bane, nukes can and will blow you right to hell without triggering mana shield first. So be willing to change things up a little
Now, following this guide, you'd have taken rocket grab first. Rocket Grab uses around half the mana you start with for one shot, so regeneration and/or flat-out more mana is useful.
Your (useful) options to that end are either Meki Pendant (Regeneration), or a Mana crystal (More mana). Spend whatever money you have left over on health potions.
Now go out and lane for awhile.
Your goal is to upgrade your starting item (Meki > Philosopher's Stone, Crystal > Goddess' Tear), get basic shoes, and start work on a Sheen.
Finish the Sheen as soon as possible, since double damage after using rocket grab/MORE Power Fist damage is insane.
Upgrade your shoes. If the other team is caster/snare heavy, go with Mercury Treads. If you think you can handle 'em, pick Berserkers'...or go the tanky route and get Ninja tabi.
Now that you have your Sheen, it's time to get a bit of cooldown reduction. There's two real options here that'll work.
The survivability option is a Frozen Heart. Extra health/mana and the attack speed reduction to nearby enemies makes it rather easier to handle things.
The Damage option is a Nashor's Tooth. If you hadn't noticed, Blitzcrank's attacks without overdrive are pretty slow, you get one more for the speed than the AP.
Start getting components together to upgrade your Sheen to a Trinity Force. I usually start with a Phage (Slow and extra attack damage, yo)
By the time you finish your Trinity Force, odds are people are either grouping up to avoid being rocket fist'd, or simply fail to die when you've dragged them off. This is the time to work on damage.
I rather like Madred's Bloodrazor for that, especially when fighting a tank-heavy team. You could also probably go for a phantom dancer/Infinity Edge/Black Cleaver as well.
If fighting a caster heavy team, replace this with a banshee's veil.
Your final item build should look something like this:
Philosopher's Stone/Goddess Tear (You could sell 'em off in favor of something else, or upgrade the goddess' tear to Archangel staff...Most games seem to be over by then, though)
Shoes (Whichever you went with)
Frozen Heart/Nashor's Tooth
Madred's Bloodrazor/Banshee Veil
Whichever of the previous two you didn't get.
It's rather simple, really. Take potshots with Rocket Grab (Especially from the bushes, if the enemy is careless), add Power Fist once it's available, and use Overdrive to chase 'em down if they survive.
Ult whenever it seems appropriate, though the passive is fully capable of scoring kills if you've softened up the opponent enough (so don't waste it)
Late game your strategy changes: You should have shoes of some sort and maximum Overdrive. This combination makes you just about the fastest non-Teemo hero there is, so rush in, Power fist whoever looks most careless, and use rocket grab when they attempt running for it.
Use Power Fist whenever it's off cooldown, it's good for last-hitting, backhanding melee heroes who are in your lane, and is just overall good.
You're going to have to learn to aim Rocket Grab if you follow this guide, remember that it fires Blitzcrank's right hand, which affects trajectory slightly.
Get a lane-buddy. One with either heals or a snare is preferred.
Heroes You Will Hate
Ryze: While you can rocket grab while Prisoned, he will quite likely have nuked away most of your health and part of your mana barrier (if not outright broken it) by the time it wears off.
Morganna: For similar reasons (though replace 'nuked off health' with 'let teammates maul you'). Also black shield blocks rocket grabbing.
Twitch: Everyone hates Twitch.
Everyone with Wit's End: Mana Burn hurts with your excess supply, which in turn makes mana barrier weak.
Veigar: Primordial Burst will ruin your day.
Kassadin: As with Wit's End: Anything that steals your mana is an evil, evil bastard. Plus his spells are rather painful depending on his build.
People with Flash (Or Flash-like abilities): They escape Power Fist Justice far, far too easily.
Ideal Laning Partners
Twitch: Rocket Grab is an incredible setup, as you should know already. Pair it up with an ambush from Twitch and you'll likely kill the target.
Evelynn: Same reasons, opening stun on someone you just grabbed is gold.
Taric: Heals for you, Dazzle stun for the opponent, Radiance for pushing over towers.
Soraka: Anything that gives you mana is awesome.
Alistar: Ideally, you rocket grab someone to you, then Alistar punts them into the tower for a kill. All healing is good healing.
Ryze: Immobilizing a rocket grabbed target is always good
Morganna: Early game, rooting and rocket grab is doom to enemy heroes. Late game, Blitzcrank is weak to them himself, which means black shield is much appreciated.
Teemo: Rocket Grab into the shrooms
Heimerdinger: Rocket grab into the turrets
Overall, coordinate yourself with a teammate for excellent results. Dragging the squishy enemy to you allows for all sorts of double-teamy goodness. Blitz is best with a helper.
That's all I've got. Let the criticism begin, I guess >.>