My favorite was Gargomon since I was young. What a cute and powerful looking Digimon, and it has Gatling arms too. But, a new Digimon popped up called Namakemon which was so weird and cool looking. It gave Gargomon a run for its money. Hydramon is my favorite Mega Digimon now, and it was worth the wait. An awesome design for a plant monster, it’s like an extension of Blossomon, but now a dragon hydra hydrangea monster. Though it doesn’t look like something that should be in the air. Other ones I like include Numemon, Sunflowmon, and Pagumon.
Probably cliche, but WarGreymon, then followed closely by Chronomon (Holy) and Calumon in no particular order :)