I tend to shoot outdoors / landscapes / nature significantly more than I do portraits. I was looking at the Tamron (i think) 50-500 - because it seems so damn fun. But that's the kind of upward range I like. Even something with xx-400 range. I just don't want a 400mm lens that is less versatile than a great zoom. If there's something exceptional in that price range, but still over, I welcome the suggestion, but I won't be buying a $12k lens.
Answers here are all over the map, partly because people seem to be telling you what they'd buy and not what you ought to but, but partly because your description of what you are shooting is ambiguous. So let me take a couple of guesses.
"outdoor/landscape/nature" can mean a lot of things. Since you talk about the super-telephoto range lenses it really sounds like you're thinking wildlife first, landscape second. So your primary lens need is super-telephoto (> 200mm) but you need wide angle as well. That pretty much sums up my photography -- lots of birds, lots of critters, some landscape work, and people only by accident or when forced to.
If that's what you do, you can't do it with one lens, not with any quality. You can cover the range well for $2500. If you are willing to spend some more, you can cover it VERY well.
For a long time I shot with a 24-105 (which others have suggested) and the 100-400. You can buy both of those new for right around $2500. If you buy the 24-105 used (and really good copies are available at a good price) you can hit that number. you have to be careful about buying the 100-400 used; it's a lens that can lose sharpness as it ages and gets bumped around. Newer units seem both sharper and more resistant to that, older units can meet Canon's standards but not be as good. So be careful and test before committing. Or buy new.
Willing to spend more for a better set? Again, I like the 24-105. Buy a 70-200 F4 to cover the medium telephoto, and then buy the 300 F4 and put a 1.4teleconverter on it (I like that better than the 400mm lenses. a bit more flexibility and I think it's sharper with faster AF). That'll run you about $3500 new, but some used shopping will get that down to about $3000. The 300F4+1.4x is a great bird/critter lens, about the best you can do without spending LOTS of money.
Next step up: 24-105, and the 70-200 F2.8 IS II (NOT the older IS). Add a 2.0x III teleconverter, and that turns it into a 140-500 F5.6. That combo has speed, fast AF and sharpness to die for; it's well beyond what the 300F4 can do and blows away the 100-400. It'll also cost you $4000 new for the pieces and the lenses weigh like a brick. (NONE of these combos are light). However, the wquality of the imagery is superb. It's what I upgraded to a few months ago, and the 70-200 just blows me away. It's new enough that there is effectively no used market for it, so it's hard ot shave price here, but I love the results.
The sigma 50-500 gets raves frmo some and criticism from others. It's soft at the telephoto end, but all zooms soften at the telephoto end. Whether it's TOO soft, you'd need to test, but it's an option. But you still need a wide angle, 50mm isn't wide enough for good landscape work. You'll find 24mm on a 7D will make you wish for even wider stuff, trust me on that. (hint: resist the temptation and learn to stitch panoramas. Take shots with the 24-105 on vertical -- spend the money on a good tripod and head instead of pushing yourself to something like a 10-20mm, or consider adding a full-sized sensor like a 6D.
Before you spend money on ANY of this -- rent it or borrow it, take it out and try it. Test it hard. You really, really don't want to spend this kind of money on a lens you decide you don't like. Take some time, figure out what works for you, THEN buy. If you're new to all of this, seriously consider buying the lower end lenses and consider upgrading in a few years; the used market can be your friend here for selling off and upgrading.
Also before you consider buying really expensive lenses (like 500mm or larger, $5K or more expensive beasts), realize that (like avoiding buying the wider stuff by going to panoramas) you can crop images off of a 7D, or you can consider buying a higher megapixel body for a LOT less than these big-nasty lenses, and with some thought and some cropping turn out very high quality work. Modern digital technology gives you options beyond just buying more expensive lenses if you learn how to take advantage of what you can do.
Lots of options here. Take your time, rent and test. Figure out which lenses work best for you. Don't spend the money first and regret the purchase. A rental or two to avoid a mistake is a great investment. Or find photogs around you that you can borrow a lens from or go out shooting with and try things out. These aren't trivial investments. you wouldn't (I hope) buy a car without a test drive. These lenses are like that.
A top of the line everyday lens for a top of the line crop sensor, with specialty in landscape/outdoor rather than portraits, you will be looking for something like these:
- Canon 17-35 2.8L II: For convenience, and wide angle shots.
- Canon EF 24-70mm f/2.8L II: If you sometimes do need to zoom a bit more. Can serve as a portrait lens if it turns out you need it, occasionally.
- Canon 24 1.4L II: For amazing quality. Primes are creme de la creme.
- Canon EF 28-300mm f/3.5-5.6L IS USM. If you really need a superzoom for wildlife shots with more versatile everyday use.
- Canon 300mm F4L For wildlife shots, weigh significantly less than a high quality super zoom and being a prime with IS, it will allow you to take amazing photos.
A huge bazooka superzoom can hardly be considered an everyday lens. The fourth option above weighs 3.7 pounds and is as long as a sheet of A4 paper.
All the lenses cost between 1300-2500$, so they are within your budget.
Considering that according to your comments you are looking for a telephoto lens, I see two choices if you want the flexibility of a zoom lens. One is the Canon EF 100-400mm F/4.5-5.6L IS USM Lens which is well regarded but on the dim side. The other is the Sigma 120-300mm F/2.8 EX DG OS APO HSM which is slightly above your budget but gives you a bright constant aperture. You can shoot in lower light and get a more shallow depth-of-field. In between the was the discontinued Sigma 100-300mm F/4 which is truly excellent and you may be able to find used. Unlike the other two, this one is not stabilized.
The Canon 17-55mm F/2.8 IS USM would be my pick because I tend to favor wide-angle and it would be very suitable for landscape and outdoor shooting. It is also stabilized which helps for low-light hand-held photography as long as you are not expected to see subjects in motion to be sharp.
However, it does fall short for portraits, so maybe a 24-105mm F/4L IS USM would suit you better. It depends if you tend to shoot wide-angle or not and is really a personal decision. A better quality one but not as long would be the 24-70mm F/2.8L IS USM II which is truly exceptional.
To get a long reach and high quality, you really have to consider more than one lens. Any of the 70-200mm lenses Canon offers are highly capable, although I would not call them everyday lenses.
The EF 70-200mm f/2.8L IS II + EF 2X III extender.
This combination gets the same Image quality (IQ) at 400mm as the EF 100-400mm f/4.5-5.6L IS. And at 100mm & 200mm (without the extender) the 70-200 is noticeably sharper at f/2.8 than the 100-400 is at f/4.5-5! Unlike some lens + extender combinations, the EF 70-200mm f/2.8L IS II + EF 2X III still focuses fast enough to shoot sports.
Without the extender the 70-200 is one of the best telephoto zooms ever produced. It has IQ that approaches prime lens quality all the way from 70mm to 200mm and all the way down to f/2.8. With the extender it holds its own with just about any other zoom in the 140-400mm range. Here are side by side comparisons of the EF 70-200mm f/2.8L IS II and the EF 100-400mm f/4-5.6L IS. Since you're shooting with a 7D, I changed the camera option for both lenses to the 60D (it has the same sensor). At 280mm the 70-200 was tested with an EF 1.4X III extender, at 400mm it was tested with the EF 2X III.