I've always felt it was Maria. I really only stan for Britney as much as her, although I stan for others too just to a lesser extent, Florence, Lana Del Rey, Cate Blanchett, Jennifer Lawrence.
My friends actually asked me this a lot in 09/10 and I always said 'Maria. It's just Britney never let's me down' even though Maria was sucking at the time. I still won't forgive Maria for smashing my phone after RG 2008.
I think it's just because I invest more energy in Maria though, I mean staying up to ridiculous hours to watch all the matches etc. but then the pay-off is bigger too. RG this year for example. I just truly admire her, she's a wonderful person through and through IMO. Plus Maria's fight on court is infinitely inspiring and the way she picks herself up after snatchings.
Plus, it's actually quite hard being a Britney fan. I mean sure she's always basically guaranteed to be successful but you just look at her and are constantly reminded of how all creative control and legal rights have been stripped from her. It's sad.
Both have had major impact and influence on me and I'll never forget either of them, it's hard to see me stanning so hard for anyone else in later years although it likely will happen. I just respect that both are fighter and have went through a lot yet still came out on top. I admire nothing more than a talented and true strong woman.
Oh, and one of the other deal breakers is Maria isn't afraid to be Bitchy. There's times when I wish Britney would read other artists who bash her for filth but she's too nice.
I know this is long but I'm basically debating it in my head as I write.
To finish, Maria definitely, but it is hard to say too. And just because it's Maria it doesn't mean my love for Britney is much less they are very close. The two of them practically take up my whole heart.
EDIT: An important point to make is that Maria is on the scene for all but a couple of months a year whereas Britney deliberately vanished from the public eye between albums which adds to the whole 'investing more time' thing.