17-40L - Landscapes. One of my favorite lenses. I love 17mm on a FF camera and really want to get a 17mm TS-E lens. I doubt it'd replace the 17-40L though. AF makes the 17-40L too usefull for me.
50mm f/1.4 - bokeh, although I'm not very impressed with the lens. My friend has one and loves it so I figured I'd give it a go. FL and f/1.4 is fun to play with, but I'm just not impressed with its performance. Everyone seems to rave about it and love it, but I'm thinking I may sell
100mm f/2.8L IS Macro - had the non L/IS and upgraded because I wanted IS and weather sealing. IQ is pretty much the same with a very slight edge to the L for colors and contrast.
70-200mm f/2.8L IS mk2 - Perfect FL for me for general walkaround use. I mainly bought it for indoor sports shooting, but it will probably become my most used lens.
300mm f/4L IS - Bought for wildlife and just to get my foot into the super tele range. After getting the 70-200 I'm thinking I won't need the 300mm anymore and will probably sell it.
2x mk3 extender - for getting 600mm and playing around. Also used on the 70-200 with great sucess IMO for a 140-400mm.
Other lenses I've owned
18-55mm IS - Upgraded to 28-135
55-250mm IS - Upgrade to 70-200
50mm f/1.8 - broke after 2 days of ownership, bought brand new.
100mm f/2.8 Macro - upgraded to the L/IS version for IS and weather sealing
28-105mm USM - first USM lens, wanted to see what all the fuss was about
28-135mm IS USM - better range than the 28-105, has IS, and better IQ
24-105mm f/4L IS - came with 5D2 kit. Don't really NEED it, and I found myself using other lenses less.
70-200mm f/4L - bought the Sigma 18-250mm OS HSM and didn't want to swap lenses all the time, so I sold it even though IQ was much better
Sigma 28-80mm macro - it was $50 and had good macro ability... What do you expect? Upgraded to the 100mm f/2.8 Macro
Sigma 18-250mm OS HSM - Bought a 1D mk2 and couldn't use the lens on it. Sold and bought another 28-135mm
Tamron 400mm f/5.6 APO - Gave to a friend as a Christmas gift after getting the 300mm f/4L IS
I will/might have at some point
17mm f/4L TS-E - I will have one. It's next on my to buy list. I really want to get started in the world of tilt/shift and 17mm is the perfect focal length for me.
24mm f/1.4L mk2 - I will have one. I love wide angle, and f/1.4 just can't be passed up. I really feel like just using the 24mm will push/challenge my photography.
24-70mm f/2.8L - probably, still unsure if I want to buy. Used a friend's and liked it, but I don't see why I'd want this around since I sold the 24-105mm. Seems like overlap.
24mm f/3.5L mk2 TS-E - possibly. Not sure if I want to buy this lens yet, but it's not out of the question.
300mm f/2.8L IS - Simply stunning lens and with the mk2 out now, the mk1 will become cheaper and within my price range.
500mm f/4L IS - MAYBE in a long time, if/when I can afford it. Trips to Australia for shooting wildlife in the bush would be awesome, or BIF at Midway, or even just a whale watching tour in the Puget Sound...
8-15mm f/4L - I'm a sucker for UWA lenses. It's so much fun. Plus the fisheye would be awesome.
135mm f/2L - possible for indoor sports, but unsure because of the 70-200mm f/2.8L IS mk2 I already own. I doubt I'd need the 135L