I've owned the 24TS, tried out the 17TS, and owned both the 16-35 v2 and the 17-40. My opinion depends on just how rich you are. If money doesn't matter, or at least matters little, then hands down, get the 17TS and 16-35 v2. The 16-35 is "killer" sharp and contrasty in the center, and depending on the QC of the particular one you buy, between good and very good on the edges and between fair and good in the extreme corners, where "un-flatness" of field rears its ugly head. A good copy of the 16-35 is always going to be better than a good copy of the 17-40, but, stopped down to, say, f/8-13, the advantage is very, very slim, except in contrast and distortion, where the 16-35 remains a little better still. The 16-35 is generally better on the wide side, from 16mm to about 28mm; at 35mm, the quality is lower. The 17-40 IQ is a little more even - not as good at 17mm as is the 16-35 at 16mm, but not as bad at 40mm as is the 16-35 is at 35mm. Also, don't under-estimate that the f/2.8 max aperture of the 16-35 helps a little with focus, both manually and in autofocus. All this means little, however, if the price of the 16-35 is prohibitive. And, the 17-40 is also a little bit smaller and lighter, but not much, and a comparative bargain.
As far as the 17TS is concerned, if you want the best extreme wide angle Canon makes, then this is it, period. It's very expensive, however, and, for landscapes, rather than exterior architecture and interior photography, I'm really not very convinced that a lens this wide is really necessary. But, if your style of shooting is one that that focal length works best for you, then run, don't walk, to the store to get it. Because of the lens's bigger projected image circle, neccessitated by the shift function, the corners of this wide angle, used with no movements, are really very, very good and very unlike any of Canon's ultra wide zooms. Of course, depending on how much swing and/or tilt you might apply in an actual exposure, the corner quality will vary quite a bit. No question though, if you need a 17mm focal length lens to be sharp across the entire frame, this is the only one to get.
The 14mm is a great lens and also better than Canon's ultra wide zooms in general IQ, down to the corners. However, it is short of the IQ performance of and, clearly inferior in versatility to the 17TS. If you need the extra angle of view that it affords, it is a great lens. But, for general travel and landscape type use, I see 14mm as potentially a lot of fun to use, but not exactly necessary. I see its use falling most into the rare times when you just plain need something as wide as possible because your back is literally to the wall, but you neither want to have the distortion of a semi-fisheye nor go to the bother of stitching a couple of 17TS images together - which yields a potentially even wider angle of view than does a single 14mm exposure. Or, when you want to use its resultingly exagerrated perspective to "make a point" in your picture. Otherwise, it's an expensive luxury that's not often neccessary for most people.
As to third party wides, I can't say because I don't own any. I would urge you to try them for yourself. If they work for you in terms of IQ, build, handling, reliability and longevity, fast and accurate focus, etc., by all means get one, because they'll always be cheaper. Cheaper and still adequate for the job at hand is always a good thing for those of us with less than unlimited resources. But, also keep in mind that a ghood "L" lens almost always holds its value better at the back end, when you may finally want to sell it years later, and you may sometimes even be able to sell it at a price higher than your original purchase price, or at least at a really good percentage of it. Good lenses, unlike camera bodies, really are, most of the time, closer to an actual "investment," even though that word is usually carelessly thrown around when purchasing almost anything that a salesman can talk you into.
Good luck,
David
