>>47576558Speed Boosting abilities (or abilities that allow you to bypass speed, see Gen 6 Gale Wings) are among the best abilities in the game for offensive Pokemon but I wouldn't say they're THE best abilities in the game.
Objectively, they're very good, but they aren't always necessary and the two primary ones (Speed Boost and Unburden) have draw backs that keep them from being too ridiculous (Speed Boost works in increments and thus won't help against scarfed Pokemon or Pokemon who are significantly faster than you if you don't have time to get going, Unburden requires you to use or lose your item and is strictly once per battle so if you switch it's gone forever, and both are bypassed by priority and are rendered moot against things that were slow anyways).
Personally, my favorite ability is Magic Bounce because of how satisfying it is to use and how fun it is to punish people with it.