For me, I'd have to say Old School X-COM (UFO Defense, TFTD, X-COM Apocalypse to an extent). I love them in a way that I just do not enjoy NewCOM, but the interface was quite possibly based on the recovered works of a 13th century scribe, and is very difficult to get used to for an unfamiliar player.
Oh and the diplomacy, they made it so much more bearable to actually deal with people and gave some indication of how they felt and what it would take to strike the deal where in old games it was just obscured such that it doesn't make any sense.
I also really liked the retinue system changeup even if it didn't really stop me from just building clone doomstacks of Tactician w/ 2 archers, crossbows and trebuchets + Red Horse Lord w/ 6 beastly Charge Cav (the red cavalry lines were all busted as hell, the melee cav was absolute pants in comparison even the peasants because the charge attack would wipe out basically any unit you wanted to attack with cav anyways) + Green or Purple lord with the heaviest infantry I can muster.
you can fix diplomacy in rtw & m2tw by using force-diplomacy to make the ai agree to reasonable terms :garf-troll: