I've used a few American Crew products, starting with pomade in college (brown label) and moving my way to Fiber (blue label) and Forming Cream (green label).
Personally, I now use Forming Cream. It gives a pretty substantial hold while still keeping hair somewhat soft once it dries (hair is still pleasantly "touchable"). The finish has a very slight healthy shine (not completely matte), but not at all an oily or greasy finish. It smells very good, and is easier to work through hair than fiber. Forming cream is really good for short to mid length hair.
Fiber has a superior matte hold. The hold is unbelievable, and it also doesn't make your hair feel hard. I think of it like a clay, but better. If you want a completely matte finish with an extremely strong hold, fiber is the way to go. However, I find it is less versatile, as it's hard to work through longer hair, and it's harder to work into and style hair in general.
To me, pomade is too greasy/oily looking, but if you're going for a shiny look, that is a decent product.
I had a friend that used American Crew Grooming Cream, but apparently it has less hold than Forming Cream.
I somehow received the Citrus Mint Clay. It smells good, but it basically tore hair out of my head and was impossible to wash out. I would avoid this at all costs.