It's really subjective. I could give you some standards, but each one is only as important as the whole and each one is only true to a point. The most important thing about nice tits is that they are on a great person. I would rather see the subpar tits of an otherwise attractive (by looks or personality) than a great pair on an otherwise disgusting person.
Here are some standards that I would go by in general. I've ordered them by importance, but like I said, any or all of these can either be determinative or go right out the window with the right person:
1. The person wearing them. More often than not, this makes all the difference.
2. Size. I generally prefer larger tits.
3. Shape. Usually whatever shape the tits are is a reason why I like them ("I love how round they are!" "I love how they hang!" etc.).
4. Turgidity. I generally prefer firmer tits.
5. Bounciness. I don't know if this relates to the tits or the activities of the person wearing them, but bouncing tits attract my attention, which in turn makes me like them.
6. Celebration. If someone loves their tits, loves to play with their tits, or loves to have me see or play with their tits, then I will generally love their tits as well. An otherwise odd or subpar pair of tits becomes awesome when it's presented as this fun thing that the girl is excited about.
7. Availability. I don't generally get excited about tits I can't see. If you've got the cleavage going on, then I'll probably think more highly of them. If you are wearing a thick sweater, I might not be thinking about them at all.
I've never really put that much thought into the science of enjoying boobs, but as I keep thinking about it, I'm realizing that there is a lot to the subject. In that regard, I hope this helps and feel free to ask me for clarification on any of what I've posted!
Everyohne has a difference preference I'm sure even though many may well be about the same, but mine are: meduim perky boobs! :-P Like now overly huge nor tiny by any means, and they stick out nice in a tight tank. lol!
Small perky boobs are really good. Big perky boobs are really good. Pear shaped boobs or boobs that protrude out like missles instead of mellons are also really nice (and interesting).
Big sagging boobs might not be good or they might be just fine. So do you know the pencil test to see if a girl's boobs are sagging? I will remind you. I girl puts a pencil under her boob and if the boob can hold up the pencil then her boobs are at least sagging somewhat and she failed the test. If a girl fails the pencil test it's still OK; she can still have really nice tits: just as long as she can't hold a textbook up with them.
I am not into bee stings; at least have a full A cup. I don't think I'm really picky about boobs.
I'm a girl, not a guy, but I'm so weighing in on this one! I think a nice rack is REAL, first off. Then, size doesn't matter so much as the way it looks on the body. Like, a small breast can look fine on a thin, skinny girl, but a girl with some body to her needs a little more, like a C. I don't think that superhuge boobs look right at all, though. I also think they should be rounded, and definitely up on the chest, perky, instead of trying to sag a lot (and I know a lot who say sagging isn't fightable, but it is. Underwire and some plastic surgeries can fix that!).
It's probably crazy for me as a female to say this, but still. Most of us females will admit to checking out another girl, usually because we're comparing her to ourselves or others we've seen, and that's what I think looks best.