League of Legends: which champion should I buy?!?

Well it depends on your play style. Since you said Renekton you probably like to be aggressive in lane and tanky late. The champ i would recommend is Nasus his damage late game is very high if you stack your q's. You cant really find your best champion without trying them out i would say you should buy Caitlin, try her out if you don't like her refund. But some chars i recommend you use a variety of champions to get better.

Here is a list of champions that are fighter/tanks that would be good to try.

(Shyvanna) Great in lane,jungle

(Nasus) Need to practice stacking q's to be effective but once good almost unbeatable.

(Renekton) He is a great fighter in lane and late game with great tank potential.

Those are 3 you should try out there are many more but just giving tips.

If you like to play adc than try that out with Caitlin she is one of the easier champions to use as adc.

But like i said before if you want to get better than you should play a variety, also because playing repetitive can get boring so try new things.

They are both very good champs and you can't go wrong with either, however if you've already used re nekton and liked him you should probably stick with him