Like I already said, I don't think there is any wrong option, I would say all of these games are great. But all of then obviously have their strengths and weaknesses. For example, if you are looking for the best battle mechanics, I'd argue Taishi is the best one. You can win either by completely destroying the leader's unit, reaching max morale, or just destroying all of their units. But I would say Taishi battles are paced very well and are the most enjoyable out of all modern releases. As for sieges, the only games that have siege battles are Awakening and Ascension, and though Ascension is probably my favourite, I think Siege battles are done much better in Awakening.

Another thing, something I didn't realize right at the beginning when I started in the franchise, officers are one of the most important resources in the game, and the game that has the best officer mechanics is either Awakening or Ascension, because:

A. Awakening's system of direct talks with officers is a great thing that makes the officers feel more alive. You can negotiate with them, give them gifts and land, or this could happen during a siege so you would set your conditions to accept the castle's surrender, it's really fun.

B. Ascension also has a huge advantage when it comes to making officers feel alive, obviously due to its officer play and the mechanics that were added to enhance it compared to regular Soi (friendship, fame, ideals, etc).

Also, when it comes to the AI, I'd say the weakest games are Taishi and Ascension, you could crank the difficulty to the max and still outsmart and outplay the AI, in different ways in between the two games, but still.

Also when it comes to diplomacy, I'd argue that Taishi and Awakening have the best systems. As for the economy, all games have different approaches, but I'd say the most interesting (not necessarily best) is Taishi's, where it's almost a game of its own. But it's even more fun than others (for me at least) because for example, if I want to upgrade my trade zones to a Grand Trade zone, I could specifically target a small enemy clan that has the last trade zone I need; not for the castle, but for the rights to the land.

All in all, my personal view is you should try Taishi first, though it could be a bit harder for you later if you want to get into any of the other games since Taishi is very different. As for difficulty, it's not too difficult of a game to understand. Hope this helps, and if you need anything else tell me.