Animals. Mostly Cats.
Random / Temporary Memes
Double rainbow, David at dentist, ice bucket challenge, autotunes. Very unpredictable. Browsing thru this will show you the breadth.
“YouTubers” with some kind of recurring schtick. Jenna Marbles, Hank Green et al.
Original versions; parodies; lip dubs and other amateur copies.
Accidents, pranks, car crashes, stupid stunts…
Clips, remixes, fake trailers, interesting comparisons
Superimposed at start of most videos, but also popular in their own right
GoPro and more.
Of popular genres or TV shows…
How To and Tutorials
Basic life skills. Cooking, grooming, make-up…
Academic or Pop Science
TV Clips or Custom Short Form Videos
Haul videos, product reviews & comparisons.
Gamers playing games.