When choosing a lens to go on your DSLR there are many issues to be considered first, the best lens available may not be the best suited for you. The debate over the Canon 400mm f5.6 L USM and the 100-400mm f4.5-5.6 IS L USM is cropping up time and again and if some people could answer themselves a few simple questions first then it could be a simple decision.
Using the above two as an example and with my own thoughts at present I hope this will help.
My advice (I'll leave some points alone) is to consider what you intend to take photos of. The 400mm prime is widely accepted as a very good birding lens, light, portable and very sharp. I suspect mainly down to the few glass elements in it's construction which also gives it very fast focussing too. On the down side close-focussing is 3m and it lacks IS. I would be very happy to own this lens and know it'd take great shots but my choice now is for the 100-400mm. This is because I want a lens capable of taking photos of subjects other than just birds. I have a set of ext. tubes with which I hope to have a go at doing some macro work and I feel the prime with it's 3m min. focus would be unsuitable for this purpose. Also the zoom capability of the 100-400mm does allow that flexibilty when working with closer subjects (something I now appreciate all the more) and even can do some landscape shots at a pinch. I've also grown rather used to having IS and I'm now loathe to be without it!
In short my choice has been made because I want (can only afford) one lens that can do a range of tasks and of course is of sufficient optical quality and enough reach to be a good birding lens too.
Other people may have different criteria by which they choose their lens or may have enough money/muscles for a range of lenses and so they can offer their opinions on these matters.
So please, no more questions as to which one's the 'best lens in the world', it doesn't exist! Decide which one is the best for you.
Goodnight everyone.
Adam