For me it's my old Marshall chorus pedal...I just can't do without it, especially for clean sounds (blame Rush and the Police). It also has a setting with no modulation that I've grown to really like for distorted leads, as it really thickens and warms up the sound without adding any warble.
I'd say delay and reverb as well, but for the ambience I tend to use the POD's internal sounds rather than stompboxes, just for convenience's sake. My DD3's mainly for U2-style effects.
Teese wizard wah. I use it in 75% of my solos. Usually not as a wah, but as a fixed filter a la Michael Schenker.