My favorite and first custom scenery pack is Jcat's ScreamWorx:
http://www.trackworx.com/screamworx.html awesome!8)

Moby Steel Jungle, it is better than Steelworx. Steelworx doesn't really let your play around with the supports that much. Like how in MSJ, you can make custom supports how you want it.

Moby's Steel Jungle and Gadgets Coaster Goodies. Just having those 2 sets makes you coaster unbelivably amazing.

For coaster accessories, I would say the ones RCTCB mentioned, plus ATH Catwalks.

For buildings, M&P Coal Mine and Rockside Lagoon.

WHY THE HELL DO YOU BUMP AND NOT NOTICE D= For better safety don't go too far in the forums.

It's okay by me because it's a good thread.

Also CTR_Sind's stuff is great for buildings.