For me it's gotta be any type of FIC maze challenge. It hasn't appeared recently (I think its last appearance was in GC) but that's my go-to challenge to watch on YouTube. It has produced some iconic moments like in Cagayan with Woo winning and the very close finish in HvV.
Close second is the auction reward "challenge", but I personally wouldn't consider that a challenge.
Tbh, I like a lot of the ones that require endurance and determination. So the one where they hang onto the thick, big pole is one of my favorites. (Even though it's definitely not as creative or original. I just find it interesting to watch.)
Get a Grip is a good one. I think Last Gasp might be my overall favorite. Some of my other favorites are Puppet Master, House of Cards & Wrist Assured.
My favorite challenge is definitely Out on a Limb, like having the choice of nthe stations wchich one 1dt which one 2nd etc... and its giant and my favorite version is the one with slide so fun to watch