Well it certainly (as previously pointed out) depends on what you want to shoot. But the question of which IS lens to get does bring up some points I have been struggling with myself.
If you make a few assumptions (ie Canon only and L series only) then Canon seems to have some holes in the lineup in my opinion. I mention L series only, because for me (and I know this is just me), I only buy L series lenses because of my use of them in still photography as well. And in that case, the only real choice Canon gives me for an L series WITH IS is the 24-105 f4.0L IS. And it is irritation to say the least.
So, for not knowing what you want to shoot, and just assuming a general purpose lens with IS, my vote is for the 24-105 f4.0L IS.
Incidentally, I personally think this is at least one of the reasons people are eagerly awaiting either the fabled 24-70 f2.8 with IS or wishing Canon would take the 24-105 up to 2.8.
^The Canon 17-55mm f2.8 IS USM is, for all intents and purposes other than build quality, an L lens.
My favorite lens is the Canon 70-200 f/4 IS. There is simply something ineffably penetrating, highly emotive, and je ne sais quoi about the images it creates. I say 'it creates' because I've taken shots I didn't 'feel' at the moment that turned out to fit the above description.
I'm liking the 55-250 4.0-5.6 IS as a gap lens to extend the handheld reach of a 17-55 (actually I'm using the Tamron 17-50 VC). The 55-250 goes to just about 100mm at 4.5, which isn't bad at all. I can't do a steady handheld shot at a longer length than that anyway, so it's a real practical choice for me. I just can't open up as much as I'd like in low light.