Personally, I have a Neo Geo with the MVS converter. It saved me a ton of money in the long run since MVS carts are far cheaper than the AES ones, plus it's nice to know I can use the carts in a Neo Geo arcade cabinet someday if I ever want to buy/build one.
As for games to check out, Pulstar, Last Resort and Blazing Star are definitely great in terms of shooters (though, as we've already discussed,
Viewpoint is my all-time favorite for that system). I'd also recommend Waku Waku 7, as it's a fun fighting game with some truly bizarre characters - you can see my review of it
HERE. Another 2-player game I love is Nightmare In The Dark - you can see my review of it
HERE.
On a semi-related note, if you enjoyed Metal Slug, check out "In The Hunt" - same kind of game, same developers, only you're in a submarine. It's not on the Neo Geo, but you can find an arcade ROM for it I'm sure.