It's not an easy call at all. Yes, WI improved after Lloyd took over, but his captaincy also coincided with the emergence of Richards, Greenidge, Roberts, Holding and then a spate of great fast bowlers. How exactly could he go wrong?
Looking at Brearley, his tenure immediately followed two series against the great Aus side of the mid 70's and one against the almost as great WI side of 1976, so results were always likely to improve, especially as a lot of the sides he faced were weakened by Packer. Plus he had the sense to get out before England faced WI again. Plus he inherited Willis at his peak and Botham whilst he still took it seriously.
I rate Ian Chappell highly, but it did help having Lillee & Thomson to unleash against England in 1974/75. And perhaps people forget that Greg was actually in charge when Aus beat a very good WI side 5-1 12 months later.
It's not only about improving results, imo - there's too many other variables that can affect that. It's about adding value, which must take into account the players at your disposal and the quality of opposition faced. During my time, I'd plump for Imran, Howarth & Vaughan. And possibly Ganguly. Before my time, everyone seems to really rate Benaud, and I see no reason to disagree.
EDIT
Looking back at some of the previous comments, I'd have to agree with what was said about Border. Beyond what's already been said, his handling of Warne as he started his career was absolutely brilliant at a time when SW really needed that sort of backing.
Hutton's also a decent shout. He did benefit from a truckload of great players emerging soon after he took over, but they were young during his tenure and he handled them brilliantly to win in Aus (apart from doing a Nasser at Brisbane, of course).
Illingworth was OK, but a bit like Brearley in that he was lucky regarding the standard of opposition. He probably had the best opening batters and best fast bowler in the world at a time when standards weren't great, excluding SA, who he never had to face.