My always ons are the Source Audio EQ2, UltraWave (if not clean compressed, then very dirty with compression, perhaps expansion), Atlas Compressor, and usually a touch of room reverb on the Collider. When I engage more effects, especially the C4 synth stuff, I'll use a bigger reverb, but always like having at least a touch to open the sound up.
And reading this thread and noting how specific some of the answers were made me wonder how many people are using completely song-specific effects, and how many will engage different effects or different presets on the same effect to mix up the song? I experiment with different sounds every time on most songs especially those utilizing a lot of effects, partly because I wasn't using MIDI to create scenes, and I also couldn't remember which synth preset of effect I used for song x or y last time. Plus, I just enjoy making each performance of a song unique to keep from being bored with the repetition of some of them.