The age requirements? They're all rated mature. That's either 17 or 18. Since you already have MK1, MK2, and MKDA, I suggest getting MK Trilogy (the real Ultimate MK3)and MK Gold (updated MK4). That way you won't be missing out on characters, etc.

For me Its a tie between MK1 and Ultm. MK3.

I guess it would have to be Ultimate arcade for me, the home versions(MKT included) had issues. MK2 feels rather boring now. MK4-Gold hasn't aged very well. MK1 is slow an feels old.

Just classics in general I'd say UMK3 because it was the best version that was in the arcades. If MKT was in an arcade I would say that. For the home systems I'd say MKT for PSX definately is. Its got a lot of characters and is pretty well balanced with the exception of Noob Saibot and MK2 Kung Lao. And this is only for PSX. I dislike the N64 version. The characters are not as well balanced because the limitations on a large number of moves were lacking allowing for a huge number of cheap and easy infinites and huge damage combos. MKT for PSX is balanced much more like UMK3 for the arcade. Also, I like the characters for MKT for PSX that aren't in N64.

MK2 is probably the best just because it has the best graphics, gameplay, and sounds.