I don't have an absolute preference. They all work great in specific contexts and rooms.

I'd love to try One18 again, but it's been a while. They were great and I didn't feel I was missing low frequency information. I was coming from PSI A21M so the One18 were the logical, natural choice. The bass is not hyped nor exagerated on either of those models / brands. It just reaches low enough to make infromed decisions.

Then I moved into a smaller room and now I'm happy with the One15 at nearfield distance. They are faster, more opiniated and you cannot bend your mixes against the nature of the material you are presented with, if that makes sense. They guide me like a dog. I'm happy to have the Flexbase, though for some thumping energy, and for clients when tracking.

I keep the One12 for 5.1 duties or remote sessions, stuff like that (not often do I get gigs requiring 5.1 though). The One12 are more like a mini One18 in their character, than a smaller One15 IME. They are relaxed and natural sounding. You wouldn't believe the amount of lows those little guys are outputting without farting.