What games you should look to play first? These are some of my favorite exclusives personally:
-Forza Horizon 3
-Ori and the Blind Forest
Since you say you like MMO's, I'm not sure there are any MMO exclusives on XB1 right now, but Black Desert Online is coming to XB1 as a timed exclusive in the future, so definitely keep an eye on that. Obviously, if you get an X, it will be the best platform to play multiplats on since it has higher resolution than PS4 Pro and sometime other graphical improvements for most multiplats.