Vatus mobility and unique skills make him incredibly fun to play - but take this with a grain of salt as I haven't actually used him and that is based solely off of shooting range experience.
Barik is a very versatile frontline and can be played with multiple styles for both point and off point frontlining - very fun.
Mal'Damba admittedly should probably be further down as he is harder to use, however - once you get used to him, much like Barik, he is a very versatile champion who is very fun to use.
Octavia is unique in the sense that even when you're playing badly you can still just have fun knowing you are helping your team just by being there.
Androxus and Maeve are both fun champions with good mobility and more so for Androxus than Maeve - good versatility.
Atlas is another champion who harder to get used to. He is very much so like Damba in the sense that once you've learned his abilities fairly well he is fun to play with.
Viktor and Tyra are both versatile, low skill required champions you can always use when you want to just relax and have a solid game.
Seris is a versatile support much like Damba is without the skill needed. Another champion who you can just have fun with whenever however.
Vora is basically a stabby spider man with built in anti-stealth. What's not to like?
Grohk is very fun in the sense that you can always be good at anything with him, its incredibly rare to have a truly great game with him but it is always fun.
Fernando and Kahn could be both higher and lower on this list depending on playstyles. It is incredible how much Fernando's shield can help a team, and Khan is the king of pissing opponents off along side Ash.
Evie is incredibly fun (SS maybe) to use once you get the skill needed to be great with her. However, due to just how much skill she takes I cant put her higher.
Rei is basically a walmart Seris. I guess thats fun. (I haven't used Rei yet.)
Drogoz is always capable of making a difference in any game.
Talus could have been higher, but with the addition of Vatu to the game Talus' ult has lost nearly all value and I feel that takes away from the champion.
Ruckus, Ying and BK are all A+ due to the simple fact that the funnest games are so good you can overlook the bad ones.
Strix while being a boring Kinessa to some people is still a very fun champion to use that is, in my opinion, on the same level as Kinessa.
Corvus is enjoyable to use but isn't as versatile as he should be, which takes away from him overall.
Grover is much the same as Corvus.
Everything in A- is a fun champion but one that simply doesn't have the same x facor that the champions above them do. Never a bad game, but never outstandingly fun too.
This is the spot for champions who, at the start, are incredibly fun. However, these champions all get boring after a few games and then you dont play them again for a week or two.
These are champions who are fun in the sense that you can always win, but winning will always feel cheap or like a drag be it due to auto aim skills or just not much action from the champion.
These guys just simply aren't fun to use.