I've only ever played RotTK 6, 10, and 11. For a good nice experience with PSX - style graphics, go with 6. It has all of the basics, good game. 10 is the best, you can start as a lowly general and rise through the ranks to become the emperor, while 11 has real time combat and focuses much more heavily on strategy.
I'd say the best is 10, except it has one glaring flaw, bad AI (but they all do really). In that game as mentioned, you can roleplay many more things than just warfare. You can be a wondering scholar, lending your talent to a particular state, or be a general, or turn like the wind as different states rise or fall, or commit to one faction for the rest of the game. I actually ended up playing for Dong Zhuo (the evil dude). What's amazing is the attention to detail, every local general I recruited or met turned out to be from that same area in history too. You start really getting into the lore, I really liked how brilliant my character had been in history. The game also has a lot of nice touches like a hero & item & event database.