On the whole I found the Avast forums OK but there was one guy - can't remember his username but he had a parachute as an avatar, who tended to criticize posters and be very unhelpful and patronizing instead of providing constructive advice.
Although probably not as friendly as MT I always found Bleeping Computer very helpful - got me out of a hole years ago before I knew anything about malware when I got infected with a fake antivirus masquerading as Microsoft Security Essentials (the genuine MSE which I was using at the time was next to useless).
And although they're not purely a security forum, SevenForums is also excellent. I've never been a member of Wilders but they seem pretty good.
So my Top 3:
1st= MT, sevenforums
3rd Bleeping Computer
Already attended other but did not stay long and abandoned, including forums you have here in Brazil, MT was the only one I liked, I think much more organized and compared to others I visited, seem to have the best people, all are well educated, I find it essential for a forum!
A while back I posted that there are some sites that i have not visited in so long I forgot my login so I removed them. Well the one you mention is one of those and yes MT is far more organized and user friendly than many of the others, and you have to admit some much cooler staff here as well