135 no IS, and don't go by what I do...
I'm weird with my glass choices.
ultra wide angle. For you that's something in the 10mm range to the 20 something range, so the sigma 10-20 or canon 10-22... but:
OK, I'd suggest you hold off a bit longer. You need to settle into a shooting style for what and how you shoot. Once you've settled into a routine then you'll know which end you use more and what's holding you back. If you're always at 18mm then a 10-2x would be smart. If you're always at 55 then a tele zoom would be the next logical option.
For me I'd always go with a UWA zoom as my second lens after the kit. But looking at someone like matt, he'd get something longer I bet. We shoot differently so we use different lenses. Not better, just different.
Figure out what you like to shoot and how you like to shoot it and then buy a lens accordingly.