just buy the master chief collection, you will love it. every halo from 1-4 (with remastered editions) and you can play each one online. i still pop it in and play from time to time, they are classics. do not question this one I am 100% sure you will love it.
as for what games you might like... its hard to say. there are very few straight FPS's anymore. I myself, am a Fallout 4 lover. its FPS but with a lot of RPG elements.
here are some shooter-style games that are considered some of the best at the moment:
Rainbow 6 Siege
Overwatch
The Witcher (this isnt really a shooter but is a great game)
Fallout 4
PUBG (very popular, if you can find some people to play with you will love it)
Fortnite Battle Royale (kind of a cartoon-y pubg that is free and a bit smaller scale)
there are lots more, honestly just going to the store section of the xbox and browsing what is popular is a good idea. they have TONS of videos and you can even scroll down on the specific games page and watch a live stream of someone playing to get a feel for it before you buy (this is kinda lame but a good idea before you drop some $$$)