In terms of trophies (post Mercer) then only Book comes in with a shout with the league cup and runners up in the league in 2 successive seasons.
Keegan and Royle did have great sides, but two of those best seasons were in the 2nd tier of English footbball and one in the 3rd tier!
Horton's football was good to watch with the likes of Beagrie, Quinn, Rosler, Walsh etc - but the team just avoided relegation in both his seasons.
Kendal - I believe, might have won a trophy with City, FA Cup or League cup had he stayed, but the football wasn't pretty. Reid did well on the back of Kendal in 2 successive 5th places before losing the plot with Sam Ellis....
McNeill did fantastic on a shoestring, the shoestring that ultimately led him to leave for Villa after 7 games... and both City and Villa went down at the end of the season!
Machin and Ball I shall pass on.....
So back to the top, it's Tony Book for me (he also managed the Youth Team to the youth cup victory in 86 too ;)