While there's lots of nice chips in Vegas (Aria and the MGM are pretty, forgot what the material is like though), my favorite are the chips from the Vic in London. They're always clean, relatively new, and are quite heavy, which threw me off the first time. As far as I remember, all of the 25 and up chips also have coin inlays.
Dirtiest in my experience are probably Fox Poker Club in Soho, London. I hated Pechanga's chips in California too. Cheap, ugly, dirty chips and I hated the room: high rake, max buy-in of 50 BB on 1/2, only used $1 chips on the lower limit games (1/2)... quite fishy though.