Original Merlin. The most badass wizard ever.
Eehhh. He's (when he's male, anyway. Didn't the original spend a few years female?) got points for being one of the first (Sorta'. Kinda'.), but definitely not the most badass one
ever. Just somewhere near-ish the top. I've definitely ran into several just in fanfiction (Including alternate Merlins!) that could top the original merlin pretty easily.
Don't mistake me, dude(occasionally -tte) gets mad props, but he's not on the top of the totem pole(s)

The Merlin. King Arthur's Merlin.
I was King Arthur too. I duelled a superpowerful being of pure magic for like six turns, quite successfully I might add.
There's dozens of both of those! I dub thee
Arturia, tiny blonde woman. Still an impeccable swordsperson, though!