Without having imperative data on all available lenses, I have used a number of Macro lenses from different Manufacturers, including Nikon, Canon and Olympus. Let me just point out something important in your quest:
None of the "all-around" macro lenses mention here, be it the Nikon 105/2.8, Canon 100/2.8, long-discontinued Olympus 90/2.0, or any of the Sigma/Tamron/Tokina/Kiron lenses which can focus at infinity, will be among the "best" at 1:1. All of these lense are optimised for image reproduction around 1/10th life size (give or take), and use clever optics and floating elements to give very good performance on either ends, i.e. infinity, as well as 1:1. Often, such an all-rounder is the "best" lens because it can be used in so many way. For this use-cases, all of these very similar lenses are great, and usually comes down to brand loyalty.
If, however, you are talking about perfect optical performance at 1:1, you have to start looking at the dedicated 1:1 lenseds, and for my money there is no beter performer than the Olympus OM 80mm f/4 lens, wich is perfectly optimised for 1:1, and can go from 1:2 to 2:1. Your other option for this class of performance is to use a good enlarging lens from Schneider-Kreuznach or Rodenstock. Al of these lenses need to be mounted on a bellows (and Olympus has a nifty adjustable metal extension tube which is great with the 80mm lens) - they cannot be mounted directly ont he camera. The amount of extension controls the magnification.
As far as I know, the absolute all-time best resolving macro lenses are the Ultra Micro Nikkor lenses made by Nikon. Some of these are very exotic, including focusing different (invisible) wavelengths of light, thus not all that useful for general photography (these are industrial lenses). You will likely never find, or be able to afford, these lenses, but some (slightly over-enthusiastic, if you ask me) info on these lenses can be found Dead Link Removed
When looking at more readily available lenses (aka. Canon/Nikon) I believe you won't find better than Canon's MP-E 65mm f/2.8 lens, which ranges from 1:1 to 5:1 magnification. A wonderful lens, although the Olympus 80mm f/4 ont he Olympus variable extension tube handles just like it, with helical fine-focusing thrown in to boot.
So, when simply discussing the "best" 1:1 lens, I am sure it would be one of the Ultra Micro Nikkors. However, for slide duplication, you won't easily find a better setup than an Olympus 80mm f/4, mounted on an OM Bellows with the slide duplication setup. You can easily mount it on your EOS with an OM-EOS adaptor, it's very easy to use (has marks on where to focus the bellows for 1:1 magnification for the 80mm lens, etc). You could get this whole setup, I think, for about $700 - the bellows and slide attachment go for about $150 if you are lucky.
Of course, if you want to duplicate (do you mean "digitise"?) slides, you can get a top-of-the range flatbed scanner for that amount, which will likely outperform a macro-lens + bellows + digital SLR for slide digitisation.
Novoflex made the bellows and other parts for Leica for many years, quality is superb.
