New response, from years in the future!
Milky is my tracker of choice, making XM my format of choice. It's stable, it looks nice, it feels nice.
All the trackers with a FT2-like or ProTracker-like layout are fairly comfy to me, and I know most of the XM effects from heart.
I like the IT format more [more features], but I'm not a fan of IT/Schism's layout [seriously, pages? gah!], Cheesetracker *refuses to save* and crashes alot [but it's fairly usable, it's a shame the song I tracked in it couldn't be saved], and I really don't like OpenMPT.
Chibitracker is a bit nicer than the rest of the pack [still IT-like with the layout, but done a bit better], but it's also horrifically buggy [read: it segfaults too much to trust my work to].
In terms of other trackers I use, I'll kick something out with Renoise. Shame there isn't a replayer for this other than Renoise itself. Other than that, I'll use FamiTracker if I need to join an NSF OHC. Rarely, I'll dabble in Klystrack, or if I'm feeling really old-school, I'll fire up AHX, but that almost never happens. And then, sometimes, I'll pull out RMT to do some Atari POKEY stuff.
old response below:
MilkyTracker, namely since it was the most intuitive tracker that I found. (and my only other choice was ModPlug, and I HATED how it felt...)
And I use XM format ( since it's what MilkyTracker saves... ^_^;;; )