I started torrenting in 2005. My first tracker was a native one, which is now offline because the government took action against most of the torrent sites in the country where I live.
Since then I've found many different sites with different content and user bases. But there's still one tracker where I'd want to be a member. It's FTWR.
Your question would probably be 'Why do you think you will be a good member?'. Well, I don't know exactly and I can't be sure of that, but what I'm sure is that I always seed the files I download for a long time and also I have a good upload speed. I can be active in the forums. And I also use irc often, so I can hang out with the peeps on there. In my opinion that are the qualities a good tracker user should have.
I can give you some links to my profiles on some sites I'm a member of.
Thanks for reading.