Ok so I have been saving Ip this week to grab up on Kog'Maw, but from the posts ive been reading in forums idk if hes worth buying or not. I have pretty much any runebook combo for any champ. And 23 champs bought from ip (minus one or two from rp) I know my champs in and out pretty well. I just want some constructive input from these three diff champs owners (pros/cons) on which would be good to get and learn.

Honestly, your best bet is to wait and see how much badder they are nerfing ezrael, and how much they are buffing kogmaw. The safe bet would be karthus because he is OK, and doesn't seem to getting changed in the near future.

All three are strong candidates. Kog'maw is most likely to be the "safest" buy, since he's horribly underrated. Ezreal's gotten the community spotlight and is being nerfed "to the ground" next patch with his range and blink getting destroyed. Karthus is pretty strong and would be a fairly good purchase as well. Regardless, expect to be underwhelmed by a 6300 champ for the first 15-20 games you play with them, until you get down their combos and skills.