It's not that hard for me to pick today at least, but 3 years ago I would have easily said it was Rachel, whilst her breasts weren't ever as large as Leanne's (maybe for a year if you've seen a couple of photoshoots of them together), size isn't everything, I think they always looked really good on her body compared to Leanne, heck even when she gained that pandemic weight/bust she still looked really good with her semi-slim figure, but her nipples have always been really pointy and they give her breasts just that extra level of attractiveness and character that a lot of other larger breasted women haven't been able to reach, Maserati was close but they didn't get super large and Milena's nipples were mostly small ish, with most of her nipple being shrouded her areolas.
Tessa has great boobs overall, but I don't think they're quite on the same level as the other three in the poll, as she's a few cup sizes away from them and has never been pregnant (but that may be a good thing in some way as that will probably keep them from getting really saggy).
But ever since Lana has gotten pregnant, her tits have ballooned up like crazy, she was an M cup I believe when she was pregnant the first time but seems to be hanging around an L (so slightly smaller than Rachel currently) however her nipples have really taken shape, and look just as good (maybe even better, which would have been a crazy thing to say years ago) as Rachel's.
The pic below of Rachel probably doesn't do her justice, but I tried to find a similar one to the angle of Lana's photo, which I was gobsmacked when I saw it for the first time. Probably the best way to compare their nipples at their greatest proportion would be videos when there is heavy nipple play and stimulation involved to see them at their absolute longest.
Lana's pregnancy did her wonders, It seems like her nipples probably won't be shrinking in size.