Civ4 is my favorite Civ game.
For me, Civ4 no expansions is lots of fun, maybe my favourite even, but felt a bit lacking in content and mechanics. Civ4: Beyond the Sword was rife with neat mechanics and content, but the AI would only ever fight through massive deathball stacks. This made the game extremely frustrating for me, especially when two nations were at war with me. Originally, AI would attack with pockets of units coming from different angles, pillaging your stuff when they could. That was cool.
I have not played Civ5 and Civ6, though I did briefly play Civ: Beyond Earth which used the Civ5 engine.