If you're looking for a unique aeg...get anything other than an M4. they're cool, but everybody and their cousin has one. A lot of the choice has to be made by you and depends on a lot of things. What price range are you looking at? Any series of gun you particulary like? Are you tall, short, etc (I'm pretty tall with long arms. Me using a P90 is laughable, I can't hold it comfortably). What size gun do you want? Modern era or historical?
For shorter guns, you have MP5s, UMPs, some variants of AKs (folding stock), and whatnot. Longer guns, you have the M14, G3 styles, M16s... You also have the historical guns, like the Thompson and Garand. Narrow down your range of guns, give us a few different ypes you might be interested in, and I'm sure we could tell you who makes reliable models of them and where to go to get them.