Well if you want to try the original nes game you could play that. Or play the more well known snes game earthbound or the newer gba game mother 3. I haven't played earthbound that much but i don't think you need to play one to play the others in terms of story. They also have different characters.
When I ran it on my SNES Flashcart for the first time it brought up anti piracy issues(a lot of games have done this because I dumped them improperly). ucon64 fixed that up though. I have heard the game can delete your save file(near the end I heard) if it detects its pirated or something). Just use savestates if you want to be safe. As for your original question I would go Earthbound(Mother 2).
i got all of them, used wikipedia to find all the correct names!
theres english translation out there for each game!
then i downloaded them and played them through my wii, using an emulator channel! pretty phat on a big screen!
I must say i found them amazing, almost as addictive as PKM!
Definitely play Mother 2/Earthbound before you play Mother 3. There are some direct plot connections, and while you dont need Mother 2 to understand anything, it will increase your appreciation for the later half of the game.
Mother 1 is pretty close to Mother 2, while direct connections exist I dont think it'd be a problem to play 2 before 1 because of how similar their plots are.
Although I went from 2 to 1 and then 3, it might actually be a good idea to play them in order. The end of 1 sets up the story for 2, and the end of 2 sets up the story for 3, even though all 3 games are years apart from each other timeline wise. You'll also get a better understanding of the main antagonist in 2 if you beat 1 first.