For myself, I really only use about 2-3 jailbreak apps, but I consider them essential. SBSettings is a MUST for me. I couldn't imagine any iPhone without it. Throw a nice theme on it, like iRetina+ and you're good to go. I also have TetherMe so I can occasionally tether. I also have iFile just to have the ability to work with my phone in case I need to change anything...
FakeClockUp is another amazing app which speeds up the animations in iOS4 by up to 10x. Just makes everything run a whole lot faster IMO. Also like RemoveBG. It's an SBSettings toggle that will close all of your apps currently running in the background.