For a mage, Cassiopeia is a fun mid and can dominate lane if you're skilled enough to land her skill shots and her stun correctly.
For a jungler: Lee Sin (who can also solo top efficiently, which I prefer) has fast jungle clear times and can gank really well. Maokai also has great clear times, tanks so much damage, and makes a great ganker with the root from his W.
For an AD carry: Caitlyn has an amazing range and is good for harass, while being easy to last hit minions with because of her fast attack animation.
For a tank: Probably Maokai again, because he deals a surprising amount of damage for someone who can take such a beating, and his ult is great in team fights, because all of the damage soaked up inside of the circle equals more damage when the ult is deactivated. Great initiator with his W root.
For support: Honestly, I don't find many viable supports in this IP bracket. I can help out an AD carry with Oriana because of her slow/speedup ability, her sheild, and her knockback. She also has a decent range.
Overall, there's not really a "most fun" 4800 champ, but this bracket has some of the most fun champs you can play.