Treat yourself to the Canon 24-105mm f/4L IS USM - a much maligned lens on various fora on the net, but believe me it is a superb lens for landscape work.
24 to 105 is a very nice wide/zoom combination for shooting landscapes and it's as sharp as a Canon L series lens should be, although it does have some slight vignetting when wide open, but that's easily fixed with modern RAW software tools and goes away entirely if you use the lens for stitching. It is also sold at a lower price than recommended retail as a 'white box' version, due to it often being sold as part of a 5D MKxx kit, but then split off from the body and sold separately.
I have quite a few lenses (don't we all), but the 24-105 is my walk about lens and is what I have been using for probably 80% of my shots over the last 7 or 8 years and it's the lens that always goes back onto my camera before it goes back into the bag. You will not be disappointed with this lens, but if you do decide to get one, then buy a new one and not second hand, as they can have a fault on the older versions with the dreaded "err 01" problem, which can cost quite a few quid to get repaired (I paid around £300 last year including shipping and insurance from the Isle of Skye back down to the mainland to get mine repaired)
see here for a video showing the error happening, but I had been using the lens a lot and in all weathers and for many years travelling with it to various places around the planet.
I also have the TS-E 24mm f/3.5 L II, which is stunningly sharp when used correctly, but which can be a bit tricky to get the hang of at first, but well worth the effort when you do and it also works quite well with a 1.4x extender, which then makes it around a 35mm TS-E, so two primes for the price of one, but I very rarely use the extender as 24mm is just right for most landscape shooting I find, when all you really want to do is use the tilt to change the plane of focus and use a wider aperture. I also bought the 16 -35mm 2.8 L for landscape shooting many years ago, but I rarely take it out with me any more, as there is just too much distortion and fringing, although I am sure the MkII would be better, I have found I can do better with stitching than I can with single super wide shots, so never felt the need to upgrade it.
I am sure there will be some that disagree with me on the 24-105, as it is only f/4 - f/5.6, which I know isn't the fastest lens on the planet, but for landscape work and at these kind of
prices, you really cannot go wrong with this L series lens and with modern cameras being able to use higher ISO speeds without too much visible noise, then f/4 - f/5.6 is no longer such a big issue. So yes the 24 - 105 isn’t perfect, but for a good all-round work horse of a lens for landscape shooting, I don’t think it can be beat, it's not the longest or the sharpest or the widest Canon lens, it's just all-round good and when coupled with the 5D MKxx live view focussing, it can help produce RAW files as pin sharp as you could ever wish for, even when printed out very large - I have recently sold some images mounted on canvas at 48x32 inch shot through the 24 - 105.
In fact to sum up I would put it like this, if I had the misfortune to drop the 24 - 105 and smashed it beyond repair, I would go straight on-line and buy a new one, before I even started sweeping up the debris from the old one.
Dave