Munchlax and Shuckle are tied for 1st with a base of 5, and Trapinch, Bonsly, and Ferroseed for 2nd with 10. Following are:
15:
Silcoon
Cascoon
Igglybuff
Wooper
Cleffa
Roggenrola
Pineco
Wooper
Fongus
20:
Jigglypuff
Geodude
Sentret
Togepi
Slugma
Wurmple
Azurill
Torkoal
Boldore
Solosis
Escavalier
Ferrothorn
Litwick
See for yourself:
Double Click on Speed- Top is the slowest