The best fisheye available for Canon (and any system) is the new ef 8-15mm f4 L fisheye. It's the best of the bunch by a long way. It combines full circular 8mm through to 16mm and every point inbetween and some fun zoom bursts too. In my opinion, save and buy the best and let it be the last fisheye you ever buy.
The best fisheye for you....is dependent on your budget and is a personal choice.
Well that is a purely personal opinion, after all I use my fisheye more than most and I have no desire for the 8-15. I don't want the gimmick circular image with horrific CA once, let alone pay for it and carry it always.
As for IQ at 15mm, my Canon prime is very high quality (my testing showed it outperformed two different Canon 14mm lenses even when defished) and all aberrations are easily corrected; and it shoots at f2.8 for half the money and less weight than the zoom f4. Just because it doesn't have a red ring does not mean it isn't the best tool for the job. The 85 f1.8 is another very good example.
For me the zoom is a curiosity that I have no intention of buying, it has no additional functionality for my picture taking than the prime does and is not worth the addition cost.
As I said, functionally the zoom has no equal and is the best fisheye currently available, fact end of...no more discussion. All other choices are personal based on shooting requirements and what you are already using.
I'm suprised to hear you say that an 8mm fish is a gimmic, after all we are talking about fisheye lenses here....they are all low use one trick pony type of lenses....although the zoom is a little more functional. But they are all gimmic lenses.
I've tried most fisheyes on the market and sold many. An 8mm circular fish can be a lot of fun and to disregard it's photographic possibilities is a little narrow minded in my opinion. To say that there's lots of CA makes me wonder if you've even used one. The two copies of the Sigma 15mm fish I've owned had slightly better optics to the old Canon 15mm fish I had. But the newer f4 Canon Zoom has less than either of them, it's still got a tad of CA but it's a lot lower than the primes. Oh and by the way, it's weather sealed...another common L feature which often gets over looked.
The 85mm f1.8 is a fantastic lens. But I sold my copy for an 85mm f1.2 L with offered me even more creativity and light gathering ability than the f1.8 version. I chose it for that reason and not for it's red ring. Although some people seem to have an Anti-L thang going on....generally the L lenses are a bit better in most areas (build, AF speed, weather sealing, robustness and often in IQ). But for a lot of photographers (non pro) they don't need those extra features or cost. I see so many 70-200/f2.8's about these days where the photographer looks like they have only handled a camera for a few weeks....poor shooting technique and awful posture and hand position. Holding their camera and not the lens like someone holding a pint of Guiness...oh boy!
Well it might have more functionality than any other fisheye "fact end of......", but if you have no use for that functionality it is worse than useless. If I was still shooting APS-H it would be good, if you have ff and crop cameras it is a no brainer too, but I don't, I am FF only which means I get a 15mm full frame fisheye and an 8mm circular fisheye. I hate the circular fisheye look and have no interest in it, as for CA I meant on the edge of the frame, not across it, look at the edges of nearly every single circular image here http://www.northlight-images.co.uk/reviews/lenses/canon_ef8-15f4l.html
If I wanted to take pictures of rainbows I would be a landscape shooter!
Obviously if somebody wants a full frame fisheye and a circular one, one lens that does both makes sense, similarly if they have mixed sensor sizes the zoom makes sense, if, like me, you just have full frame sensors and are only interested in the full frame fisheye framing then the zoom is not the best buy, the 15mm Canon is, fact end of...no more discussion.
As for the 85, sure the 1.2 is a "better" portrait lens, but try shooting badly illuminated gym basketball in AF with one, in that instance the 1.8 wipes the floor with the 1.2.
As for the full frame fisheye being a one trick pony, well you obviously see a smaller envelope than me. Here is a shot from a Canon 15mm fisheye that you would have missed living in your smaller gimmick lens meme, I didn't, I saw outside it. A full frame fisheye can be used in all sorts of situations and I have found it to be very flexible, unlike a circular fisheye which really does give you one, very compromised, image.