Want a good ranged middle champ. Which one is the best?

Heimerdinger is a unique champion and is very fun to play. However, he requires a lot more skill than the other two choices because he is, at the moment, underpowered.

My experience with Varus is limited, but his Q looks promising as a poke tool and his passive should make farming large groups easier. He is free this week, try him out.

Brand is a common mid and is a strong choice. As an AP caster you will be squishy however. (But not as bad as the squish master himself, heimerdinger)

Out of the three, Brand is the one I would choose. Of course, you need to pick what you team needs.

Brand is pretty much a "nuke combo" insta killer of everything that doesn't have a billion HP, and then goes on some substantial cool down.

Varus 'can' have some significant sustain, and great poke potential, when played right.
I play him a lot, and can usually own a lane until ultimates open up.
Then people with strong gap closers get tough to handle.
Built well, he can dish some serious damage as well, though, and adds significant more utility than Brand (multi root, slow + heal reduction, and a nice snipe for runners).

I have never played Heim, so I have no real feedback there.

For Mid, I would say don't go Varus. He may be viable, but he is more so as an ADC.

That leaves us with two. Brand is better in general, but I feel only Jax and Riven snowball as hard as Heimerdinger. His ganks can be unreal if you become a roaming mid. But Brands damage alone is above average, and his skills stack very nicely together. Personally, I feel the world needs more Heimerdingers, but either or is a good choice.

Varus is pretty good. Farm good, and your q can hit just about any mid champ. His ult has a pretty low cooldown and if the mid enemy overextends, you can pretty much get a kill.

Brand takes a bit of skill. You have to remember to use his abilities in the correct order in order to use him. If you don't, you won't really carry.

Heimerdinger does a lot of damage, has good lane control, pretty good cc (can be more reliable than Brand's), and is good in team fights. I find Heimerdinger better than the other two but of course, he's one of my mains and the other two I don't really play, so its a bit of a bias opinion. His grenade also gives vision if you have trouble aiming it. He's safe from almost any jungler and counters Yi pretty amazingly. He's a safe choice against many champs though very squishy.

I main Heimerdinger. So obviously, I would tell you to get Heimerdinger (heavy bias). Here is why.

He has excellent lane presence because of his turrets, and he wrecks any melee carries early to mid game. FIghting close to enemies is beneficial to him since it is easier to land his grenade and rockets tend to aim better. That being said, he is the squishiest champion in the game. (I start with Ruby Crystal as my first item for the health. Then turn it into Rod of Ages)

His turrets are downright terrifying early game. He can put out a lot of damage with a single turret level 1, provided the enemy is stupid/your jungler can stun him in range. His rockets may seem a little weak, but with enough AP they shred mid game since most people don't build too much Mres. His grenade is great if YOU CAN LAND IT. If you can't, there goes your combo of stunning people near your turrets.

Item suggestions: Rod of Ages for health and mana (he has INSANE mana costs early game)
Ionians (Personal choice, I like frequent turrets and I get Void Staff anyways)
Void Staff
Rabadon's
Zhonya's
If you want, Rylai's

A lot of peeps use Liandra's Torment, but it isn't that good on Heim. If his turrets procced it like Singed Q did, but it only works on rockets and grenade (as far as I know)

tl;dr Heimerdinger is amazing against close range enemies, takes significant skill to play as he is UP at the moment. Recommend.