I don't know about them being the best, but they're nice. What's the name they're attached to?
As for the topic at hand (in hand?), I prefer at least a C with large(ish) areolae and nips but a lot of it depends on the entire body package. From recent searches, examples of that range are Felecia to Autumn Austin to Lana Lotts.
Like'em big but not too big (between DD and FF is fine) and round.
I don't really care about the nipples.
Natural or not ? Well, If they are natural, it's better but fake, as long as they are nice and not too big compared to the girl's -I mean, FF tits don't fit to a 90Ibs girl- fake tits are fine.
I couldn't care any less, just so long as they're fucking huge. My rule of thumb is this: from a side view, if the tits stick out farther than the stomach, it's alright. :2 cents::dunno: