[speeddemon]

Kog is probably the most versatile, you can go AD, on hit, or AP and still do well, but he also has the highest skillcap among those four, since he's squishy, slow, and has a pathetic escape mechanism (and landing his ult consistently can take a while to get used to, sniping the stragglers even moreso). His passive can be pretty fun, it's almost as annoying as a karth ult.

Trynd is, well, Trynd. I don't think it needs more explanation. It can be fun rofling people ridiculously hard, until you realize you're playing Tryndamere and don't really deserve any respect for it. Regardless, when I'm on a losing streak it's always a nice pick me up to play him for a few matches.

Fiora is fun to jungle, not so fun to lane. Her gap closer feels like a waste of time, honestly, but it does it's job. Her kit as a whole just feels kinda boring, imo. Ult is kinda fun but can be so detrimental if they realize who you're on. She's easy to pick up, but mastering riposte can take a few matches.

Volibear is ridiculously strong early on, and is great for getting first blood. His ult is flashy and feels very powerful if you build right, otherwise you bank on nuking them with Frenzy, which is equally as fun. Typically people go tank or bruiser on him, which is nice with the frenzies and is better for ranked play, but honestly I had much more fun with an on hit build. His ult pretty much annihilated the enemy team every time, and the passive provides nice sustainability in a team fight even though you're squishier on the whole than a tanky bear.

/2cents

P.S. Since you have 8k you could buy both Trynd and one of the other three, although if you did that I'd hold off on Fiora because buying two AD carries is terrible for enjoying diverse gameplay.