B'day are the roots of most of the rest of her career, I call it her career puberty, which sounds odd, but it basically introduced us to a lot of things she perfected, extended etc. today. It is one of my favourite ever records, but I wouldn't call it the best in most departments. Bey showed immense energy during the performances and the Beyoncé experience is praised as the top favourite tour by a lot of Hive, but vocally she growled a lot during that era, and even though I don't mind that, she has grown to a much better, controlled and dosed vocalist, which blossomed into the 4 album. visually she introduced us to her first visual project the video anthology, a lot of great videos, but during the Selftitled era she perfected what she started during the B'Day era, and elevated it during the Lemonade era. Sasha was born during the B'Day era, the beast she presented on stage during that era, got molded into what she later called Sasha Fierce, which made her a commercial monster during that era.
btw no disrespect to DIL, which delivered the amazing hit singles and an amazing start to her career, but she really got into her own as performer and artist during B'Day and evolved with the eras after.
Her 'best' album probably isn't even recorded yet, but from her current albums, I think it would be a close call between 4, Selftitled and Lemonade, since all 3 focus on different aspects and have different strengths.