Been watching county cricket for about 25 years now and in that time...the best player is hard to say, but the best in their roles for their respective clubs would have to be Wasim Akram, Darren Lehmann, Mushtaq Ahmed and Shane Warne. In each case, the players concerned fitted in, as one other poster put it, like locals and became part of the team rather than a highly paid gun for hire, and in each case the club was better for them.
Obviously, there are a few others who were quite amazing to boot - Sylvester Clarke at Surrey was one of the most fearsome sights, as was a young Allan Donald at Warwickshire, and I remember marvelling at a 16 year old Sachin Tendulkar from the boundary edge at Headingley, but to me the four above did something just a little more.
Beyond that, there are the Kolpakkers. Three stand out for me as worthy of mention - Michael DiVenuto and Dale Benkenstein, for all the work they did taking Durham from a new ish county to the very top of the pile, and a less auspicious name in Claude Henderson, who has been wonderful for the youngtsers now coming through at Leicestershire.