If we're talking about sets we own, probably a tie between the Bohrok, the Boxor, Makuta, the Vahki, Nidhiki, The Barraki, Nocturn, Maxilos and Spinax, Chirox, Kirop, Krika, the Zesk, the Vorox, and Fero and Skirmix.
For sets we don't own, the 2001 Rahi (minus the Manas), the Bahrag, Keetongu, Brutaka, Gadunka, Hydraxon, Vultraz, Icarax, The Skopio xv 1.