Hi,California ebony tarantula
If this is your first foray into macro photography, then I'm glad you've ruled out the MP-E 65mm. It's a very cool lens, does things no other macro lens can do on its own, but is a serious and experienced macro shooter's tool. It's manual focus only and very high magnification, higher than a lot of people find easy and fun to experiment with.
To answer your question, well it sort of depends upon what you want to shoot and how you will be using the lens...
For outdoor shooting, general purpose, flowers and bugs and such, I recommend a lens in the 90mm, 100mm or 105mm focal lengths, for use on a crop camera like yours. The reason is simple... these focal lengths give you some working distance, so you won't scare away small critters, might avoid getting bitten or stung by some of them. With inanimate subjects, there's less chance of throwing accidental shadow over it, too, thanks to the extra working distance. Also, these lenses aren't so long as to be difficult to be difficult to hand hold, longer lenses are more likely to require a tripod or at least a monopod to get a good, steady shot.
On the other hand, for indoor use... studio work and other more controlled setups, mostly with inanimate subjects, a shorter lens in the 50mm to 60mm range, perhaps even as short as 35mm might be useful. One reason is just the opposite of the above... Often with studio macro shooting it's helpful if you can reach out and rearrange the subject while keeping your eye to the viewfinder. So the shorter lens might be useful. Much of this type of macro work is done on a tripod and/or with supplementary lighting of some sort, so camera shake might not be a big concern. OTOH, with inanimate subjects and a tripod, longer shutter speeds, mirror lockup, a remote release and/or self-timer delay might be used.
There are longer macro lenses available... 150mm and 180mm in particular. With some particularly nasty critters, a bit of additional working distance can come in handy!
EF 180mm f3.5L lens. EOS-3 camera with Ektachrome E200 film. Handheld, 550EX fill flash (on off camera shoe cord and diffused). Settings unrecorded. Film scanned with Nikon 4000 ED.
With longer macro focal lengths, you end up stopping down a lot in search of enough depth of field. It gets very shallow with long focal lengths:Golden bee
EF 180mm f3.5L lens. EOS-3 camera with Ektachrome E200 film. Handheld, avail. light. Settings unrecorded. Film scanned with Nikon 4000 ED.
There are other considerations.
For one, a lot of people like to use their macro lenses for portraits, too. For that and other "dual purpose", you might want reasonably quick auto focus and an appropriate focal length. 50mm, 60mm are nice lengths. 90mm too. 100mm is starting to get a little long, but still is quite usable. Also, it's often desirable to have a large aperture for portraiture... most macro lenses are f2.8, a few are f3.5 or f4. Recently Tamron introduced a 60mm f2.0 macro... sounds interesting! I'd like to try it some time.
Also, a 50mm or 60mm lens can be a lot more compact.... taking up less space in your camera bag and a little lighter to carry around.
Personally I use a monopod or tripod a lot for macro shooting. For that, it's nice to have a tripod mounting ring on the lens. Only a couple 100mm... only Canon's, AFAIK... can be fitted with one optionally. Most 150mm and 180mm come with a tripod mounting ring.
Macro lenses have to move their focusing group a long, long way to focus all the way from infinity to 1:1 magnification... So speed of focus can be an important factor. USM lenses are faster, typically (Sigma offers some lenses with similar HSM).
Personally, I shoot most macro using manual focus techniques, so AF speed and performance is more of a concern to me when I'm using my lenses for non-macro shots.
Note, too, that your 7D has a unique feature for macro work. With a certain Canon macro lenses, the camera recognizes them and when focusing within a certain distance, if using AI Servo it will switch into a special mode that resamples subject distance four times as often as usual. In a sense, this is like a form of image stabilization (though it's just along the one axis) and should help make hand held shots a little easier. Frankly, I haven't explored it much with my 7Ds because, well, for one thing I didn't know about it and, for another, I've rarely thought of using AI Servo to shoot macro. But I intend to try it now. No other Canon camera has this feature (yet... the 1DX might) and the Canon macro lenses that I know will work in this way are: EF-S 60mm, 100mm USM, 100mm L/IS, 180/3.5L. I'm not sure if the 50mm "compact macro/ 1:2" lens will (doubt it, it's not a USM lens).
Also, many macro lenses "grow" in length tremendously, while focusing all the way from infinity to 1:1 magnification. This can change the balance of the lense... and cuts into your working distance (which is measured from the film/sensor plane, by the way). A few (including Canon's 100mm lenses) are "IF" or Internal Focus designs, which means they don't change length. But they start out larger and heavier to be able to provide this feature.
Your camera can utilize either EF (full frame) or EF-S (crop sensor) lenses, so you have quite a few to choose among. Last time I checked:
Zeiss 50/2 ZE Makro (manual focus)
Canon MP-E 65mm (manual focus, 1:1-5:1)
Zeiss 100/2 ZE Makro (manual focus)
Canon 100/2.8 USM
Canon 100/2.8L IS
I'd also add the Canon Tilt-Shift. They can be very useful for macro work, too... Particularly:
45/2.8 TS-E (manual focus)
90/2.8 TS-E (manual focus)
Personally, in my Canon kit my macro/close-up lenses are an adapted, vintage Tamron SP 90/2.5 (right front: manual focus & aperture, 1:2, about $60 invested); Canon 45mm TS-E (left front); Canon 100/2.8 USM (left rear); Canon 180/3.5L (right rear).
I use a few other macro lenses in other, vintage camera systems.
The modern gear is easier to use and usually more fun to work with!
Have fun shopping!