Depends on what type of shooting you do most
You didn't say which type of shooting you like... Practical - IDPA, IPSC... Bullseye, short range, long range, etc...
I did subgun then auto rifle. This allowed me to get into the game, and save some. 20g's will still get you some good pieces. Don't overlook the FNC, like I got. They're half the cost of an M16, are sear guns that are not married to the receiver, and are a sturdy platform. Also, the m14 should not be overlooked.
If you're going to do a subgun, WTH, buy 2 or 3 M11's. Set one up as a beater, another up like a match gun - slow fire, optical sight... and one general purpose. Sure, they ooze Ghetto, but the slow fire M11's are winning the subgun matches !
Honorable mention has to go to the BAR, but it is kind of impractical from a cost perspective. What ever you get....
spend a few dollars on accessories, spare parts and if you don't already have one, a darn good safe.