There are a few angles:
Buggy instagibbed Zoro and had Luffy on the ropes. Luffy only turned it around when Buggy was reduced to <1%. Even taking Luffy's then-strongest attack in that state left him with no lasting damage.
Buggy came back later and instagibbed Luffy, having the convenient appearance of one of the world's strongest people be Luffy's only saving grace.
On top of defeating Luffy, a goal that even Big Mom has yet to achieve, Buggy has also escaped Impel Down, put together a New World Ready crew in minutes (a feat that took Luffy years), and got the WG to recognize him via Shichibukai Status despite having been publicly a part of an attack on their greatest military base,