Fairfax 1 has become my go to Pak so I'll get Fairfax 2 to go with it. I like and use Vintage Dry, Black Velvet, Funk, Indie and Session Percussion. I don't have the Jazz, Metal or Reel Machines Paks. I'm not a metal fan so I'll probably go Fairfax 2, Reel Machines and a Jazz Pak. What kind of music are you in to? That could narrow it down some. I'm gonna pick up a Djembe and Bongos to go with session percussion. As far a midi Paks I'll probably get Reel Machines, Jazz Grooves and American Rock but I have most of the other offerings.
You can't really go too wrong but I would take the time and listen to the Paks on the XLN site and pick stuff that works for your taste, genre, etc. I hope that helps.
Rocky