Which champion to buy next?

Which champions you'll love or hate is just personal preference. But I'll just suggest a champion that I enjoy: Trundle. God, I love Trundle.

Here's a quick rundown of why you might like the others on your list: You want to try out ADC but you want to be a "spell-slinging" ADC. Or, you like Siv HD a lot. Or, you want to try out a pretty fresh ranged midland assassin (also comes with flashy plays!). Oh, and this guy is free tomorrow, so hang on to your IP and give him a try! You want to oppress your opponents in the top lane. You want to come at them with something they rarely/never see. You like mastering champions that aren't considered popular. You want a mobile AP assassin that's not Leblanc. (shots fired) You want a good option for wrecking tanks. You like playing the frontline. You enjoy smashing things into tiny bits with a big club. You're looking for an ADC that's not all about right-clicking. You want to throw down some spells, some true damage, and some magic damage. You want an ADC with a strong escape. You love Trinity Force to a degree that some would call "disturbing".

Go for Ezreal.

Ezreal will be free starting tomorrow, just save up for a champion you want to master, like one of the 6300 champs

First identify what role or roles you want to play. That will narrow down your choices. Then just pick the champion you like best. Don't worry about the meta. Champions get buffed and nerfed all the time. You will do fine.

poppy, urgot, yorick, and veigar are all pretty good