Top 5, when factoring in feat chains like Improved Bull Rush -> Shock Trooper:
Improved Initiative
Improved Bull Rush -> Shock Trooper (and knockback perhaps)
Improved Trip -> Knockdown
Improved Sunder -> Combat Brute
Improved Feint -> Surprising Riposte
One issue with a lot of these is they don't actually have the same theme despite having "improved" in the name. Improved Bull Rush and Improved Shield Bash don't have any similar language or mechanics whatsoever, for example. The feats are generally categorized into "improved combat maneuvers" and then "other improving feats."