Elena and Bonnie.

That would be Bonnie.

& Elena

Bonnie is hands down the most mature out of all of them. Here is why:
Caroline: is pretty mature but worries a lot about superficial stuff and is too caught up in her highschool life to be worried about much else
Katherine: pretty old but still apparently doesn't have a life since all she does is screw with people's lives and steal their stuff (heh-hem moonstone? witch necklace?) and try to seduce them
Rebekah: older than all of them (SERIOUSLY OLD) and probably the least mature out of all with them. all she does is pine over Stefan and try to screw with Elena since she's jealous... >.< grow up already you're like a thousand years old ugh...
Elena: too busy trying to pick between two brothers to care about much else...unless she gets someone killed (which happens A LOT and honestly, it's always her fault, sometimes indirectly but still) and then she feels really bad for like one or two episodes then forgets about it lolol
SOOOOOO:
Bonnie is the most mature, she knows that even tho her best friend Elena has a crush on some vampire guys who SEEM nice...they are still vampires and have killed many innocents, and will kill many more in the future, especiallly just to keep one girl (Elena) alive...and that in the end, they need to be killed. In season one, Bonnie had more sense, now, she helps them but she stil knows what they are capable of and will stop them if necessary
SO THERE YOU GO!
A very lengthy explanation! Anyone care to agree or disagree with what i've said? What are your thoughts? ;)