I wouldn't be able to name just one, but I think a game that everyone should give a try is Akatsuki Blitzkampf.
That game is too much fun regardless of how deep you're into FGs.
If you have experience on them, it's a really solid game that feels rewarding, and if you're new, it's very welcoming in terms of executions and options.
Other than that, the game that I probably enjoyed the most was vanilla GGXXAC, but mostly because I played it a ton with friends and at my local arcade back in the days.
There are too many "favorite" fighting games for me!