Without EQ, overall the Airpods Max. That's an easy one. But it isn't ideal either.
With EQ, Airpods Max, Hi-X65. But to be honest EQ makes most of the headphones I possess better, to me, at bass reproduction than headphones without EQ (in other words I'd take an EQ-ed HD650 over most un-EQed headphones in terms of bass reproduction).
But crucially I lack experience with Audeze headphones, so...
.
Headphones that can't maintain a constant FR in the bass because of positional variation, leakage, or excessive sensitivity to pad wear, among other factors, are instantly disqualified in my view for "best bass". And headphones with an excessive noise floor that I can still hear when bass notes are playing are disqualified as well (QC 35 / 45 for example, too bad with EQ they're pretty neat otherwise at lower frequencies IMO).
As a general rule only active / DSPed headphones can achieve the bass response that I want
and maintain it consistently. So either EQed passive headphones resistant to leakage, or headphones with a feedback mechanism.
My subjective impressions of the bass response is heavily influenced by the rest of the spectrum, so even if two headphones have the same response below 1kHz (something that I think I can achieve to tolerances below audibility for
some of the headphones I possess - but not all), if the response above differs (and in that range it's a lot harder to achieve that below audibility thresholds), my preferences for these or those headphones in terms of bass response might come from their FR differences above 1kHz, and not necessarily different non-linear distortion characteristics.
One idea I've entertained trying to do, to avoid that pitfall, is to compare the headphones I know can reach the exact same FR below 1kHz with a low pass filter to eliminate all content above 1kHz. Haven't done so yet.