I rode soraka+corki to 1550 elo. I'm fairly convince nothing can beat this lane bot if it's played right, best case for the enemy is a tie or the jungler literally having to camp 24/7.

The problem is after the laming phase these two together really only fit into poke comps.

Well basically, at most elos your opponents are making tons of gameplay mistakes that can be 'capitalized' on.

So quite often it's best to pick the support that you personally are best able to exploit those mistakes with.

Teamcomp is much less of a factor outside of competitive play, largely because of the above circumstance.

Just about any support can be highly effective both in lane and in teamfights-- you really need to be personally good at both aspects of play to best turn that into game winning.

Blitz,just for those epic grabs that make me jump from my chair
or nunu,for being extremely annoying yeti

I'd say go with what fits your ADC. Do what you can to get them strong. As a support, you can't control how the other lanes do, so it's best that you do what you can and make your partner as strong as possible before the laning phase ends.

Some combos are:
Soraka/Janna/Lulu - Kog'maw/Vayne ~ for a babysit-to-late-game lane.
Sona - Miss Fortune ~ for a medium-range extremely strong poke and laning phase, and AoE comp in team fights later.
Taric - Graves ~ tanky, AoE burst lane.
Taric - Ezreal ~ gay couple lane.
Leona - Corki ~ uber high burst kill-lane with strong laning presence.
Blitzcrank - Vayne ~ get the late game carry fed early game so their mid game = late game and GG.
Lux - Ezreal ~ long range poke/harass, decent burst in linear AoE, relative high lane-safety compared to other comps.
Alistar/Blitzcrank/Zyra - Anyone ~ they're pretty much good with anyone.

Etc.

But what if your team has 0 tanks or initiators, leona can fix that problem

I pick for team comp usually. The only time I look for lane synergy only is if the team comp is scattered so there isn't a clear winner for team synergy.

I pick Nunu if we have Olaf/Irelia/Xin.
I pick Janna if we have Lee/Mundo/Shyv in the jungle.
I pick Morg if we have Akali/Vayne/Yi.
I pick Sona if we are facing heavy poke (Jayce, Nidalee)
I pick Taric if we have 2 diving assassins (Nocturne+Talon or something).

To be fair, that's the strategy I when picking most of the time. I pick Ezreal in the same compositon where I'd pick Nunu if I was supporting. I pick Ashe in teams where I'd pick Taric support.

I find lane synergy doesn't really exist in soloqueue simply because communication isn't at the level where you can really benefit from quickdraw+shatter timing or where you can count on Ezreal providing cover fire while you poke as Sona. As soon as any of the lanes is won by either team and a tower goes down (usually 8-9 minute mark) people start roaming and it is more important what benefit you provide for the other 3 people on your team than how well you work with your lanemate.

That's not true. Every time the enemy bot lane goes to fountain or your adc goes to fountain, you can roam. You can help your jungler with ganks and your midlane with heals/shields. You can help your jungler invade. If you're Janna, you're helping all lanes by just staying alive on the map.

I roam as leona or blitz all the time

Does the enemy team have Blitz?

If no, pick Sona.
If yes, pick Taric

Win all the things.