My first can was an AAC 7.62 SDN-6. The idea being that I could get muzzle devices for my AR15, my AR10 that I didn't own yet, and even a .22 for limited use. So, if I could only get one, I would do the same, but probably a different brand, as better mounting solutions and baffle technology has come out since the AAC. Probably a 30 caliber Dead Air or Rugged suppressor with a Keymo mount, or similar. Use it on anything 308 and under.
I'd probably do the same for pistol, though I shoot pistols suppressed way less than rifles. A 45 cal can with interchangeable end caps for use on 9mm would be cool. I ended up going with a dedicated 9mm can, though. So, a pistol can would be maybe my third pick.
For the absolute most FUN, a 22 suppressor is the bee's knees. Subsonic .22 from a suppressed bolt action will make any gun guy giggle at how stupid quiet it is. Even supersonic from a Ruger 22/45 Mark IV is a hoot.
Sorry if this isn't too specific. There are so many suppressors and so many new companies coming out with new technology, mounts, and applications that it's hard to nail down just one particular suppressor for any one case.