I'm plopping my money down on Generations. Though, I feel I'm giving Colours/Colors an unfair shaft since I've never played it, though as for Unleashed, I have too many personal gripes with it though I can completely see why people liked it.
Specifically talking about the boost mechanic in Generations, I loved how you could use it to access otherwise inaccessible areas that were usually shortcuts that I would have never found the first time around. For instance, near the start of the Chemical Plant Zone on the left side of the first wide running area lies, while tedious, a very rewarding shortcut that requires precise boosts in mid-air.
A more easier shortcut can also be found in Seaside Hill also near the beginning- jumping off the lenear platform to the right and immediately boosting as you touch the water allows you to run across the ocean and to a few platforms with a hidden spring or cannon that shoots you passed a huge platforming segment of the stage.
Unique abilities specific to a character, like Sonic and his boost, and their implentation are mostly always fun to tinker with to find different routes and passages- though, one could always cheat and look up speed runs too.