Just want to watch an interesting run. I like to watch runs that abuse AI but don't like runs that glitch the whole game with clipping and that kind of stuff.
Entertainment is extremely subjective, but here are some runs I remember enjoying in the past months that don't rely too much on glitches: Crash Bandicoot 2 any% (with game over abuse). Defunct any%. The Evil Within runs (especially Akumu / no-upgrades is damn impressive). Tekken 3 unlock all endings / 1 round. Bloodborne runs. etc.
I guess in theory, games with decently fast gameplay, good presentation, creative tactics, enough variety and minimum amount of downtime are the most universally interesting ones to watch, even for someone not familiar with the game. In that sense many classic platformers like Mega Man-games are close to the "ideal" speedgame...
In terms of fluidity of motion and skill, I really appreciate any of the Super Mario 64 categories. To use a trite phrase, whenever runners go through the Bowser stages, it is "poetry in motion." That said, most of the runs I dig DO use AI abuse, clips, glitches, etc...
You should totally watch me running Sparkster. Really, REALLY fast game, all movement tech/optimisation, no glitches whatsoever, manipulating bosses into quick kills.
You should watch Drako run Sparkster, and Star Fox 64 is a pretty comfy game if you watch Beefwellington run it. He's doing it at StR.
Definitely check out Uncharted 1: http://www.speedrun.com/uncharted1/full_game You can clip by spamming aim against any wall.
I personally like watch much more speedruns without used secret worlds because these speedruns are real.