I like to collect a little of everything, so I have happily bought stuff from all three mentioned scales.

But 3 3/4" was my first love and definitely my preference whether actual GI Joe products or compatible toylines. The selection of vehicles and playsets really is the best reason for this scale.

However if I had to start an action figure collection from scratch today I would go for 6" scale. With a full selection of great Marvel & Star Wars characters, and now some Joes as well, this would seem like the best option to me.