I go through a lot of characters, so I keep Back Off Barbas in the regular rotation. I never know when I am going to run afoul of that mutt...
Probably half of the mods were created by myself. That is why I have so many. I see something I do not like, and I make a mod. Like the one to add general stores to all the cities and towns, or to add more trainers, or improve followers (IMHO Light Step is a must for every follower, otherwise it gets really annoying have them step on every single trap in the dungeon). My Night Eye mod is one I made myself. I just turned it into a standard alteration spell that you cast like any other.
I have others that are not in the regular rotation. I created a bunch of mods that alter the races, taking away the once-a-day powers which I never use, and replacing them with constant duration effects, at lower values. But I usually only keep the one active for the race I am playing at the moment. Plus I have some other armor mods I have not had the right character to try them with yet.