[mehrkat]

I really really like lots of the PDF's

The general suggestions are good.

My favorites 5 books are

Powers Divine Favor (a really neat take on those called by the gods)

Thaumatology Ritual Path Magic (a new incredibly interesting take on magic)

Social Engineering (giving social situations a closer look)

Powerups 4 Enhancements (this takes a close look at adding enhancements to abilities. This includes a really close look at Cosmic)

GURPS Template Toolkit: Characters (this is a really close look at GMs creating templates but is also really good for players. thinking out there characters)

There are lots of others that I would recommend but I tried really hard to keep it to only 5.

I also strongly recommend Dungeon Fantasy and Monster Hunters if you want a fully worked genre complete with templates and ideas for challenges.
Dungeon Fantasy is set up to do stock fantasy whackem take there stuff.
Monster Hunters is for things like Buffy the Vampire slayer, Charmed etc.
There is also fully worked Sci-Fi setting "Tales of the Solar Patrol" (think Buck Rodgersish). I really like it but I haven't managed to make my players nibble on the game idea I had.

Action is also a series for adventure stories (Bond, Commando ...) once again I like it but haven't found the right game to really try it.