Is there a list somewhere with the most useful/fun threads on MTS (besides the sticky ones)? Threads like the Random Questions one or Essential mods are very useful and so fun to read!

I don't think there's an official list but the ones that kind of get constant/semi-constant updates and have good/fun info are:

Stupid/Random Questions
Pet Peeves
Random Thoughts
What did you Discover today?
Things You've said while playing
Things you did as a n00b
You've been playing sims 2 too much when...
Look at the awesome thing I found
How much CC is in your game
Hacks and CC you'd like to see!
Have you ever dreamed of the sims?
How do you incorporate technical glitches/errors into your stories
Funniest/Strangest thing to happen in your game
Careers and Businesses for Sims
Sims Government and Customs
Do you have any special laws

I remember awhile ago there was a thread along the lines of: "Things you shouldn't do in TS2 AKA things that cause corruption". I can't seem to find it, does that thread still exist?

I found a thread that discusses signs of hood corruption
Thread here

Also there's a general list in the Wiki of what not to do in your game to cause corruption
Wiki Page here

"Most Interesting Threads" beyond the ones Charmful listed above will vary a lot depending on the particular member looking at them... for instance, I tend to revisit the various university and campus-design threads far more than I'd guess most users do, just because that's one of my largest ongoing projects. It wouldn't make sense for most people to spend as much time there as I do, because they aren't working on the same things in their game. On a similar, but somewhat broader note, I'm very much more likely to look at Build/Buy-focused threads than I am at gameplay threads, once again owing to my own personal approach to the game, and hence my own personal approach to this site.