Mariah - Butterfly era. Because that was the time when I started realize her amazingness and she was so ****ing hot during that era.
Britney- I really did enjoy all her Eras and even if I didn't fully-enjoy some , there have been some mindblowing moments in each era.
BOMT/ The beginning , the craziness, BOMT video, buying the CD, 'Crazy' video, everybody at school have something Britney to talk about. It was THE year.
Oops/ The anticipation of the sophomore, the moment where she proved that she has what it takes to become my all time favorite artist, the shock when OOPS video came,STRONGER video, the VMAS shocking performance, her aerosmith/mary j. blige superbowl performance, the whole Nsync/Britney/Justin craziness, the energy of her performances were at their peak IMO.
Britney era/ The hottest era - look wise, the VMAS again, Her performance with MJ, the MTV DIARY!, Totally Britney Live Special, SLAVE video, the choreography in that era, the DWAD Tour, Crossroads (

), her Boys remix video, I love Rock n Roll video, her flawless PEPSI commercials. It was the best era to represent the name Britney Spears.
ITZ/ The anticipation of Britney's most grown up album, the VMAS , The Kiss, Madonna&Britney, MATM video, the preview of the album, the Britney out and about special, TOXIC video, her Grammy, ONYX Tour, Britney's hotness.
Greatest Hits/ My Prerogative video & Do Somethin'.
As for Blackout era, it had so much potential to be her best era yet but unfortunately, Britney wasn't ready for it. However; we still got some memorable moments...Album's anticipation, The debut of GIMME MORE, the pre-VMAs and the whole Criss Angel thing, the POM video,BREAK THE ICE! and most importantly, it's her best album yet.