What is she intending to shoot with this fisheye lens? What's your budget?
There are only a few options (for her 5D, the 15mm f/2.8 and 8-15mm f/4, mainly), and what you should consider is your / your wife's needs/priorities. How wide does she want? In what shooting conditions?
A fisheye is a pretty unique look, and most people have a specific look or reason to use one. Could she possibly need an ultra-wide angle (aka "UWA") lens rather than a fisheye?
Here's some information to consider for fisheye vs wide-angle lenses:
Maybe just wide angle would be more useful? Or is your wife really a fan of that fisheye-look (and/or will edit/correct it on the computer afterwards)? I'm in the latter camp, so I use the Canon 8-15mm a lot more than is typical; feel free to ask me specific questions about it if you have any.
Not sure exactly what you mean by "Best" but in my experience (Owning and working with 8mm Sigma F3.5, Samyung old fisheye converter for SLR, home made fisheye from door peeper, fisheye Nikon converter for Coolpix PNS, Peleng 8mm, Nikon nippon 8mm Kogaku f/8, and finally Rokinon 8mm F3.5, by far my best experience is from Rokinon. Peleng 8mm is also good (Built and feels like a tank) but poor coating for internal reflection and flare.
Points to remember is:
- Manual focus lens is actually Focus Free with few stop higher than wide open F, when focusing ring is left just to the left of infinity.
- Circular fisheye and distortion is excellent with 8mm as long as there is no lens sun shade/paddle attached.
- You will not use the lens very often so don't spend too much.
- One fisheye image among many other normal images is nice but too many kept together makes viewer tired. So again don't plan to invest a lot here.
- Manual focus is OK. AF chips is NOT needed for this focal length.
I suggest to get Rokinon 8mm manual lens that is $300 ish and is excellent in terms of sharpness and best coating I know for the price. BTW this lens comes under at least 5 or 6 different names, all the same nice glass. Get the feel of condition and see where she goes from there. Its very easy to work with and really fun lens. Yes it is made for crop sensor and not your 5D but then all you get as potential problem with FF, is part of sun shade paddles of the lens in corner of peripheral field of view of your image that needs to be cropped from your image in post processing. I know that some people have manually cut the peddle of lens shade to get that circular look on full sensor but I don't see it need or to scar the lens. Then if that is really needed just go for 8mm Peleng. If you find yourself in situation where you may need a better upgrade in due time and actually that is needed, you can always sell these gem lenses for close to what you paid for and look forward.