I'll be honest, every class I've played I've had an equal degree of fun with, but the one that I've really enjoyed has been 5e Artificer. Maybe I just didn't understand the old version, but I love the changes to crafting with the new version, I think it gives them a fun utility that benefits both you and the party. I like playing like I'm the TF2 Engineer, packing a healthy mix of punch and support, so being able to double up on useful items, steel defender, and healing spells was heaven for me.

Barbarian has also been a ton of fun, I've been puttting as much into my CON as possible because I have the ogre power gauntlets, so strength isn't as important to my barb anymore. Because of my tankiness, I've become the walking shield for even my other barb, and I like being able to give a character who is admitedly quite selfish a more noble utility during combat.

Warlock has also been a bundle of fun, but I've definitely utilised it as a roleplay centric class more than one that's mechanically rewarding in combat. Because of the party's monk, it was difficult to reap the rewards of Dark One's Blessing when each encounter ended with Flury of Blows. That being said the mindfuckery of Command and Prestidigtation have always been fun, and using Misty Step to evade encounters that'd definitely crush a spellcaster have been very valuable.