If you want to play modern games: PS2 
If you're looking for an oldschool console (you already have the SNES): NES or Genesis. 
Simple as that 

I agree with ganonbanned: Don't get a very specific (and very old) console like the Odyssey or Collecovision 
before getting the true classics. (with classic I don't mean old). There's nothing wrong with getting the "mainstream" consoles of back in the day first before jumping into the really rare stuff.