I don't know nearly as much as DCP and Arakynd do, so you're probably better off asking them lol; they're the MGS heads here haha.
Anyways, I can still help; first o' all, which character do you like the most? If Raiden's your guy, then I'd recommend MGS2, since a chunk of the game you play as him, though you're not a cyborg. He's also in MGS4, but you don't play as him; on the flip side he's a cyborg though, and has some pretty cool scenes.
I think that MGS1 would be a good starting off point, but if you can get MGS4, then you should be good since it goes over some if not most of the events from MGS1-3. Theres also the MGSHD collection, which comes with HD versions of MGS3, MGS: Peace Walker, and MGS2. Personally I'd recommend playing all of the MGS games if you can though lol.
Just don't expect the other MGS games to play anything like Rising haha