An MP5... As I've been issued one once, fired one a LOT, have a NON select fire version now, and could actually somewhat afford to fire it more then once a year. And of course, I'd have to go the extra $200 for a tax stamp suppressor as well.
Having fired ever full auto weapon the Army has issued (not include air craft versions) I can say they are all a joy to run though a course of fire... But for personal use, and play time, the M16 platform would not be my choice as it just is not that much fun to shoot, in auto... and Ammo is more then the 9mm.. which has to come in to play on the choice for most of us.
an M240B would be a blast as would a G3, or M249, or M2HB, or many others,,, But buy in and ammo costs would make it an at best, once a year shooter.... not worth it in my book.