Anyone know? My guess is Broly
I'd say it's dependent on the type of adapations you want.
Broly's is continuous, but there's also an obvious limit-As seen against Ikari V SSB Goku. Not to mention it's specifically fueled on rage and his opponent's own willingness to do combat. In fact, he would've tuckered out entirely against Goku had Frieza not murdered Paragus. We also don't have a proper grasp on where Broly sits at the start of the fight. He was growing in the midst of it before he even went Ikari, but I don't think it's fair to say he went from like, Saiyan Saga level to their Universal+ Base Forms. It's much more likely he simply wasn't AS strong as Base Vegeta, but still within the same general tier of strength. It also capped out pretty quickly, only stopping at SSJ before it decided to utilize the power of an Oozaru to compensate for his lack of power and allow him to catch up to SSG. Then doing so again (capping out at SSB, requiring external factors to jump up as a SSJ, and then capped out again and forcing Broly to become a LSSJ to become superior to SSJ Gogeta, and capping out again against SSB Gogeta, where he just tuckered out entirely.)
Conversely, while Black's growth is sporadic, his is specifically tied to pain and has no forseeable limit. Just from one few minute fight (arguably much less due to the fact DB Time/Fights happen ultra quickly), he jumped from SSJ2-SSJ3 Level to able to tank hits from SSB Vegeta and immediately wind him in one blow. In fact, he was much too fast for SSB Vegeta in his Base Form, and that fight gave him a whole new transformation without any extra needs (Which means he made 50x the jump Goku did as a SSJ3 to a SSG level against Beerus in base, and honestly probably a lot more, plus a transformation to enhance him further). It's also continous. Every blow goes into enhancing his power. We see this in his fight against SSJ2 Goku. He also gains more martial arts mastery (fully integrating modern Goku's Style into the style he already copied and his own), more Ki Control, etc. He can even apply things like rage to his already pain based Zenkai to get the Sickle of Sorrow, which gave him hax abilities on top of his continously evolving power.
I'd say as an overall superior boost, Broly's is better, but it's incredibly situational and you'll never actually get to use it the way you want to (covering SSB Fusion levels of Power Jumps), as well as has very clear limits, whereas Black, while it may take him slightly longer, can achieve that level (he provided MUCH of the power Merged Zamasu had, who was fighting SSB Vegito), is a more practical, limitless growth pool that involves much more into his growth rate. It also rejuvenates him, and unlike LSSJ, there's no risk of self destruction.