That's like asking whether they should have released just the first or second half or Sonic 2, or the whole thing.
Out of the 2d Sonic games not ported to the idevice here is the list I would want them in 1. Sonic Rush (a DS game but it is so awesome and probably the best Sonic game released in the last few years) 2. Sonic 3 3. Sonic and Knuckles 4. Sonic 3 5. Sonic CD