Among these 4 who should I buy?

Sona
Olaf
Heimerdinger
Pantheon

In my personal preference:

Heimerdinger > Sona > Olaf > Pantheon

lux

Heimer is really gonna be a must with his changes.

Sona is a must.

Olaf is good to have.

Pantheon is played by SOME people, but he'll be worth getting at some point I guess

the answer to these sorts of questions is always karthus.

Panth or olaf.
Tanky olaf is insane to deal with mid and late game. CC does not effect him, impossible at kill with ghost at level 6. just farms and does huge damage. Great character to have.

ps: nerf irelia.

Buy both.

I think he is in that range, buy Shaco. He has a little bit of a learning curve, but he is deadly and pretty much ungankable if you are wise with your Q

Out of those..... Olaf, Pantheon.....

But other 3150 chars? Shaco, kassadin, leblanc, lux

U should buy them all.. but start with ur lane ... top olaf
Mid karthus
Support sona.