Single favourite effect?

Sporky

Delay, because that gets you chorus, flanger and reverb for free.

clarkefan

Wah, because every time I kick it on it feels like there's tons I have to learn about how to use it right. 

mike_l

Phaser, specifically an MXR Phase 90. A touch of phase can really enhance a lick/riff/phrase. Over use and it'll have no impact. Mush like any effect.
The only thing I use regularly is dirt.

seans

Big box Deluxe Memory Man (not the newer true bypass version) - preamp and the best delay. Has to be my 'desert island' pedal.

Teetonetal

Delay. Just magical.

BintyTwanger77

Teetonetal said:

Delay. Just magical.

It can be a very therapeutic effect, bit of reverb on the amp, nice long, warm repeats, could play using that for hours.

Drew_TNBD

Delay.

BluesyDave

Plate reverb.......But I do like spring verb as well.

Babones

Distortion /Sustainer