The Distortion III, Zakk Wylde Distortion and the Classic Distortion are all very similar apparently. I should get round to breadboarding one at some point.
my favorite is, without a doubt, the Box of Mike I got from, well, MIKE. It replaced the Marshall Shredmaster clone I put together, which sounds great (was my fav for a while) and is a different kind of monster all together, but I like how it transforms my SUPERCLEAN Twin Reverb into a torn-up dirty Marshall.
I also like the sound of a Rat. I got a BYOC clone that has several clipping modes, and the LED-mode has this loud abrasiveness I love.
I also love RATs and I think LED clipping options on them are great, I've modded a few to have that option. It's a louder more open sound with a lot less gainy compression. Completely different flavour to standard Si clipping.
I've got a Boss Metal Zone and Boss DS-2 but am hankering for trying a few others out...I've got the Rat, MXR III & a MXR + on my radar of pedals to try.
My old vinyl covered SCOD is the sentimental favorite, although my beat old RAT is what's currently on my board. Depending on which head I'm using, I'll either use the RAT or just the amp.
You can set it so it's almost clean with the guitar volume backed off to 8 and gets nice and dirty as you roll up to 10 for your lead break. If you do that sort of thing.
+1 on the Crunch Box. It's VERY versatile and can go from a nice light crunch to ridiculous crunch and awesome lead tone. Definitely sounds better with the volume higher than the gain though. Sounds like Cro-Mags - Best Wishes in a box.