Quickest off the mark in the Prem IMO (no particular order):
Morahan
Watson (both)
S. Simmonds
Ibitoye
Veainu
C. Walker
T. Collins
May
100% have forgotten some.
Interestingly, I read that in 2011 Big George North "smashed" (imagine this is relatively speaking given the distances) the wales 10m and 40m records that were previously held by S. Williams, so it's not always the candidates you'd expect who test the best (even though young George was a freak).
From personal experience (both myself and my brother spent time doing pre-season, training and (almost never) playing at Exeter and Saracens respectively whilst at Uni) the players who top these tests are often surprising, and their tested speed often doesn't translate to the pitch. For example, after Ashton left Saracens it was Duncan Taylor who had the fastest 100m time - I doubt any outside observer would call that. The guy who always used to top out sprint tests at Exeter, Matt Jess, never looked the quickest on a pitch despite his 100m time being habana-esque.