You can look at the 138 people I follow: https://twitter.com/abdullahkhalids

I have a strict policy of only following people with a high signal to noise ratio (with a few exceptions). I follow only a few friends (and they don't tweet anyway). I unfollow anyone who tweets too much. I use the "don't show retweets" option for anyone who retweets random crap. I also use the "muted words" feature to get rid of political tweets. I myself only tweet things I would want to read 5 years from now.

If there was an easy way, I would write additional filters per person to mute their annoying pet peeves.

My nomination for highest signal to noise ratio is https://twitter.com/michael_nielsen

I can't, I'm in sort a niche area where there are people you've never heard of but are heroes in my field. I think this is the greatest strength of Twitter, it's not the same few celebrities. You find the people who are doing the things you want to know about. What do you do? What do you care about? Look for the leaders in those fields, and search for keywords to find conversations and you'll find other people who have a little community around it.