Some would argue that Sonic 2's bonus levels are more tubes, or perhaps rollercoasters, than slides. Those people don't even deserve to play as Tails. A good slide level isn't just about the slide, it's about who is slidin'. Sonic is raw speed to him, the whole world is a slide level.
While some games use slides as a quick diversion from the main action, Mt. Splashmore is fully 1/6 of Virtual Bart. Loosely based on a few minutes from a season 2 b-story, Mt. Splashmore challenges players to navigate Bart down a waterslide. Sometimes the game asks you to go left or right based on no information, and half of the time you will end up hitting Homer and getting bounced back. It's as if Halo made you flip a coin every 30 seconds to make sure Master Chief can keep holding the gun.
MDK stands for "Murder Death Kill," three words that aren't usually associated with slides. Maybe they should be. After you destroy a bunch of aliens with a machine gun, it's fair to take a slide-break, both in real life and in videogames. It's never explained why aliens would build slides that lead deeper and deeper into their headquarters, but then again the Goonies pirates buried their treasure at a bottom of a slide and nobody ever questioned them.