This list is going by lowest total base stats:

Gen I - Butterfree (total of 385) *Ditto (total of 288)*

Gen II - Ledian (total of 390) *Smeargle (total of 250)*

Gen III - Shedinja (total of 236)

Gen IV - Kricketune (total of 384)

Gen V - Watchdog (total of 420)

Italics are Pokemon with actual lowest total base stats, but do not evolve.

Source