You can still play any non RPG whenever you like, but you only get to have one RPG and you can not play any other for the rest of your life.

Which RPG would be your one and only? Why would you choose that one?

For me FF IV. I just have so much nastagia rapped up around it, I could play it whenever I needed my RPG fix and be happy.

Final Fantasy 7 for me. Still playing it yearly and trying to notice small details I may have missed or forgotten,

Mass Effect trilogy

I don't think I have ever actually played a proper RPG for more than a few minutes, I think Deus Ex is the nearest I have got and I haven't even finished it.

I really would like to play the Mass Effects though, so those (or just one of them if I can only pick one)

Skyrim! It might be nice to play as all the races, but I might get sick of all the same voice actors after a while

If additional mods are still allowed then ether Skyrim or Oblivion. Seriously, a couple of mods and they're completely different games.

Otherwise.... Deus Ex.

I'll go back to Everquest. It kept me hooked for 4 years before and there are 10 new expansions since I left.

Probably Earthbound. Either that or FF6. I can't decide between the two until the barrel is to my head.

Xenogears, Skies of Arcadia, or FF8.

Although, there are a ton of games I will miss. Aside from those 3:

Earthbound, Chrono Trigger, Final Fantasy 6 on SNES; If you include all epic adventure style games; Terranigma, Illusion of Time, and Soul Blazer as well.

PSX - FF7, FFT, Wild Arms 1 & 2, Suikoden 2, Dragon Quest 7, Lunar Eternal Blue Complete, Breath of Fire 4.

GB: Final Fantasy Legend 1 and 2, Mystic Quest (WAAY better than the "improved" Sword of Mana remake).

GBA - Fire Emblem, FE: Sacred Stones

DS - Dragon Quest 4, 5, 6, and 9; World Ends With You; Disgaea (best version); Radiant Historia (just awesome), etc.. 

PS2: FF10, Xenosaga trilogy, Suikoden 3, Dragon Quest 8.

At least those I will miss because I will replay all of those games at least once each in the next 5 years.

Not trying to be a dick, but I just thought about spending the rest of my life drawing magic and I almost wept.