Wah is a good choice also and my only pedal for years was a crybaby...but for me using the amp itself for gain, I would have to say delay too.
I feel with wah it's too easy to overuse it. (I'm reminded of one particular gig I attended where the guitar player used wah on every song except one -and the bass on that song had wah. Not to mention some very famous rockers who are well known for wah abuse.)
Delay gets a lot of use but seems to get more thought/planning put into its use...or maybe it's just a bit more subtle.
It looked early on that over was gonna run away with this but delay is pushing the narrative