Back in the early 90's I bought Goo, Dirty and Experimental Jet Set ..... but haven't bought anything since.

Which of their 20 or so albums should I get next?

Which should I avoid?
definatley get Daydream Nation

other than that I've also got Sonic Nurse which I rather like. I see the have a new album out for about a week or 2 now but I haven't heard it yet...
I'd say either Daydream Nation, EVOL, or Washing Machine would be good bets. I just got SYR 1-3 and I really like those, but they're all instrumental so if you're not into that kind of thing I'd say stay away for awhile.
Daydream Nation easily, Sister, and I'd reccomend Rather Ripped, which is pretty good.
I'd say get them all, in this order (Although it is hard because of how different each one sounds):

Daydream Nation
Washing Machine
Bad Moon Rising
Confusion is Sex
Sonic Youth EP

Murray Street
A Thousand Leaves
Sonic Nurse
Rather Ripped
SYR's 1-6
NYC Ghost and Flowers

The Blue is their "harder crazier" stuff, the Red is their more mellow stuff.