Fiction House (though some might argue barks first, I am partial to Planet, Fight, etc)
Barks
EC
Fawcett
Gator is the guru, but I would actually put Barks books at the bottom of the list. As much as I admire his work, Disney product in general will devalue over time due to a vastly changing kid's market and a move away from 2-D animation. And Barks books being the most expensive will devalue the most.
My list would be...
Fiction House
EC / Fawcett (these two are interchangeable to me. Both have adequate supply, yet both are extremely high quality in relation to present values.)
Barks
I come uninvited to this party, but.....
ECs (extremely cheap and reasonably, even in high grade, and there have never been warehouse finds of these, unlike Fawcetts and Fiction House, they are also relatively short easy runs to complete.
Barks stuff - mostly due to Disney's significance
Fiction House - Still some nice stuff at resonable prices, but other than Baker stuff and a few classic covers, alot of the same stuff over and over and alot of it is non descript
Fawcetts - if you have read one, you have read most of them, some classic war covers though, and the late stuff is pretty tough, especially in grade
like you need an invitation