There is only one player who would always have been universally considered to be the best in any category - Sydney Barnes as a medium pacer. Many of those who saw Hobbs considered him superior to Bradman because of his skill on bad wickets. Similarly, during Bradman's prime in the 1930's some old-timers compared him unfavorably with Trumper in terms of style. Few of those who saw Hobbs or Trumper are still alive, and as time goes by fans tend to rely more or less exclusively on statistics when rating players. That is why Ken Barrington, for example, is rated more highly in some circles than Peter May or Denis Compton, although virtually no one who saw those players live shares that view. (There is a very good article to be written about the changes in players' reputations over the years). I have no objection to using statistics when evaluating players - in fact, they are essential for that purpose - but in cricket circles (unlike baseball forums, for example) they tend to be used very crudely and mechanically with little regard for the context in which a player's feats were accomplished.
Debates on the relative merits of various players can be expected to continue. However, while we can't say that anyone is a universal choice in any category, there are some categories on which a clear consensus has been established among informed observers. This is an important qualification, because fanboys tend to be driven by flavor of the month excesses and enthusiasms of various types. The results of general Internet polls tend to be far less illuminating than the views of players, journalists and officials. In particular:
Best Batsman: Bradman - now close to a universal choice among experts. I recently read a Cricinfo thread full of comments from adoring Tendulkar fans who sought to cast doubt on Bradman's achievements.
Best All Rounder: Sobers - none of his alleged rivals garners much support for inclusion in all-time world XI's, or comes close to him in polls of players and journalists. It's not much of an exaggeration to say that the only people who doubt that Sobers is the greatest are those who have not seen him play.
Best Wicketkeeper-Batsman: Gilchrist - probably a consensus choice, although there is still debate about the relative weight to be assigned to wicketkeeping and batting ability. As mentioned in another thread, many former players seem to prefer Knott behind the stumps in their dream teams.
Best Off Spinner: Muralitharan - probably closest to Bradman in terms of being a universal choice. There are a few old-timers who would opt for Laker, but they are very much in the minority.
Best Leg Spinner: Warne - Bradman described O'Reilly as the best bowler he ever saw, but even most of O'Reilly's contemporaries acknowledge Warne as the greatest.
Best Fast Bowler: Marshall - no real consensus here. If you were to take a poll of informed opinion Marshall would probably win, but I would expect to see significant support for Lillee, McGrath and Wasim Akram.