Phase1 16Vs all had electric mirrors, but the Williams 1 didnt (cost/weight).
Heres a quick guide for you:
Williams: The original homologation special, 400 RHD cars, numbered individually and made in late 1993 (some LHD cars were numbered under 400, so some RHD cars are numbered into the early 400s). Basically, Renaultsport took advantage of FIA Rallying rules that allowed engine capacity of up to 2.0 litres when a car was homologated (at the time, they were using the 1764cc Clio 16V as the homologation car). In order to do this, they had to make 2500 road-going 2.0 Clio examples. The car was named the Williams to celebrate the Nigel Mansel/Williams/Renault F1 victories, although it was basically untouched by Williams F1 Engineering Ltd. Nevertheless, Frank Williams is among the Clio Williams more famous owners.
The original edition was based on the Phase1 Clio 16V, with the various modifications basically concentrated on making the best hot hatch of the day an even more fitting home for the new Group A/N rallying 2.0 F7R engine. The Willy also wore that unmistakable trademark combination of 449 Sports Blue bodywork and gold/silver rimmed Speeline wheels, with wider tracks filling the 16Vs already pumped-up arches and sporting a larger rear lip spoiler. The suspension and transmission were very heavily revised, with the interior getting attention too.
The distinguishing features over the Williams 2 and 3 are the Phase1 exterior/interior trim ("retro" would be kind), lack of sunroof/electric mirrors/ABS/basic sound system. None of the three editions have any mechanical differences worth mentioning.
See here:
http://williamsteam.free.fr/videos/Essai.mpghttp://williamsteam.free.fr/videos/Essai.mpg
Williams 2: Such was the public and press reception for the original Williams (see
http://www.cliosport.net/articles/williams-2.asphttp://www.cliosport.net/articles/williams-2.asp), that Renault produced a second batch of Willies in 1994: much to the anger of the original Williams owners. Still, with numbers of RHD cars at less than 400, these are again very rare. These were also Phase2 cars, which in line with the general Clio range are smoother-looking and more soundproofed cars - see elsewhere for more details. The Willy 2 gained some of the refinements of the Clio 16V lost on the original Williams, such as electric mirrors. Still no sunroof or ABS and still 449 Sports Blue in colour.
Williams 3: Renault again gave in to the pressure and produced a final edition in 1995. These cars were imited to approximately 300 RHD examples and were Phase2 models. They are distinguished by their Monaco Blue pearlescent paintwork (shared with the Clio 16V), which is a lighter colour than the 449 Sports Blue. These final editions came with an electric sunroof and with ABS as standard too (I think, or at least a very popular option).
To some extent at least, its the look and your view that decides your favourite: Willy as the original rally homologation, Willy 2 the best of both worlds and Willy 3 as the refined final edition.