What speed for a Tank is "Just Fast Enough?"

I was talking to someone about Cookie, one of the strongest but slowest Mutants in the game, and now I'm thinking about something. When I played the game, I had a high-leveled Zombie Dragon. If I recall, Zombie Dragon was big, strong, and a little faster than some of the other tanks, but still kind of slow. Yet at the same time, he obliterated a lot of mutants that were weak to his attacks. I'm starting to wonder if the reason Zombie Dragon was working for me was because he was a tank who's speed was "Just fast enough." What, in your opinion for tanks, is "just fast enough?" What are some of the best Tank mutants?