If you want to play online, the latest entry (XV) is the best. It has rollback netcode for smooth online play and crossplay so you have a bigger pool of people to fight against. It's also more beginner-friendly than earlier games, although its learning curve is still a bit steep compared to other recent fighting games. Also, the new additions to the cast, namely Isla and Dolores, are really cool.
Since you own an Xbox, I wouldn't think twice about getting it.
As for my personal favourite, I'd say KoF 2002 but it's not on Xbox.