Mew and Celebi could be obtained through glitches in RBY and GSC respectively, hm. I miss those days!
As for the DS generation, I would say Celebi or Jirachi since they're never caught in the wild (so you can't hack/warp to their location like the others!). However, Gamestop released those a few years ago. Dream World released Arceus... BW Wifi released Darkrai and Deoxys. XY is releasing Celebi so...
Back to Jirachi? However, if you have a bonus disc still, you do have an infinite supply.
What's left then, Manaphy? It was last distributed in 2007; otherwise, you have to play the Ranger game. Infinite supply of them though, I think.
Shaymin is fairly elusive (only distributed at Movie 11 or TRU and a ticket for Platinum). You can try the tweak glitch, but you run the risk of losing your save data.