WISH 36 is about game supplements...
I like supplements in general, as long as they fulfill three rules. I've qualified these for years in different ways, but I think I've oriented it along similar ideas.
The first is the rule of layout value: I will gladly pay my $1.50 per ten pages, assuming that the pages are full of readable information (no ugly backgrounds [thinking KULT 2nd Ed. and IMMORTAL here], no ridiculously wide margins [ARS MAGICA]).
The second is the rule of applicability: is it applicable to my campaign(s)? If I'm not running a scenario in that area, is it still information worth buying? This is a percentage value, generally, of how much is basic information, and how much is metaplot or other-area information. I give a basic value also to how much I feel this information should have been in the main book. I'll buy 'em for readability-value as well, but generally it has a lot of ground to make up if I'm not going to run it or scavenge it for information.
The final rule is that of portability. How portable is the adventure or idea? It's related to the applicability rule in that I want to _use_ the ideas of my supplements. MAELSTROM as a main book (by Hubris?) is portable almost in its entirety to running an interesting Courts of Chaos in my Amber games. My greatest disappointment in UNKNOWN ARMIES was its general lack of portability.
GURPS, of course, has the best supplements, because even if you don't use the system, I cannot tell you how many times I have browsed the bits at the end about, "Mixing this genre with X genre..." I've never been disappointed with a GURPS supplement, even IOU, which was basically useless to me. (I already had Teenagers From Outer Space.)
My three favourite supplements? GURPS Goblins which is a book I never, ever expect to use, unless I run that muppet one-shot I've been considering. (The LintKing pointed out once what a great match muppets and goblins were in that case.) _Shadowbeat_ was entertaining and gave a great view of what it was like to live in _Shadowrun._ I'll also do a tie for #3. The first part is one I cannot remember the name of currently, but it's effectively the demon prince handbook for Kult. It's tied with the Z'bri Sourcebook for Tribe 8 in sheer demon-y goodness.