I really dislike anything in FTB that cheaply moves you quickly, for example jetpacks and mystcraft. Jetpacks are really cheap and they also have no durability. Correct me if I'm wrong because I haven't used Mystcraft much, but doesn't it just teleport you to anywhere as long as you crafted together 9 paper or something at that place?
"If you don't like it then don't use it"
I'm playing on a server with my friends and if I didn't have a jetpack and they did, things would get really annoying.
I would say the portal gun, but that is really expensive to craft since some recent update.
(though it's pretty common to find in dungeons)