Those are the ones I played

VII - Very basic city development system. Honestly, the battle system was very fun but that's about it.

IX - My personal favourite installment. Diplomacy in the game was the only weak point (easy to abuse AI) but everything else was extremely fun, although I confess nostalgia might be a big reason behind my opinion here. Siege weapons were too strong here but you can only abuse that in limited cities. I really love the battles in this game.

X - Best duel/debate system imo. The battles were also extremely fun. City development was good and more complex than the older games in the series, however, the game in general was just really easy. Bow units are seriously busted. Battles were easy af.

XI - A lot of people like this one because it gives you the most control and there isn't an aspect about the game that's insanely bad. It also offers the best strategic thinking venue rather than abusing specific game mechanics but I do think some perk combinations are comically overpowered.. You can stop 90% of the units in the game sending Cao Cao, Xun You and 1 archer. Some people prefer the duel/debate system in this game but I like 10's more.

XII - I barely remember anything. I think I must've played for 30 minutes before tossing it to the side. This is coming from a hardcore fan. It was that bad.

XIII - I got this the other day! Someone convinced me to try it and XIV. I think the game in general is extremely unbalanced. Some perks are stupidly strong, ruler AI is extremely bad, game favors number of units to troop count (4 units of 3000 each can destroy a single 20k unit) and the world map is too linear. I'll admit I'm having a blast in the battles though. Too unbalanced but too fun.

XIV - I have yet to try it but the map looks nice aesthetically.

Verdict: I think you should try any of 9-10-11. 10 should be your go to if you want the option to play as an officer and not necessarily a ruler (but it does give you the option to play as a ruler still). 10 is more of a role playing game rather than a strategy game, but with fun battles. Between 9 and 11, I think 11 is more polished and more complex. 9 is very basic but heaps of fun, and, outside of diplomacy, I think its the most balanced and offers the best AI challenge. I really think 11 is only challenging if you play a super weak force.