I firmly believe Singularity is the best final boss in the series. Here's why.

Bayonetta 1 - Jubileus:

Jubileus is teased throughout the entire game, and if memory serves me right, Baldur fuses with the effigy of Jubileus or something. The fight takes place inside like an energy orb thing and Jubileus's attacks are so bad. You can't even get off a full combo on each limb even on the highest difficulty.

The ending climax with Queen Sheba is literally just punching Jubileus into the sun whilst avoiding the other planets. Nothing crazy.

Bayonetta 2 - Loptr (Phase 3)

Loptr's final phase is arguably the best section (both pre and post Loki removing the eyes). Baldur's speech about the power of man is just so uplifting. The true final phase AFTER Loki removes the eyes is awe inspiring. Baldur and Bayonetta fusing to create Omne is animated beautifully and the drop kick on Loptr is so funny. This ends in Loptr's physical form flying into the mouth of Jeanne's Gammorah.

Bayonetta 3 - Singularity

Whilst I'm not personally a fan of Singularity himself, I truly believe that he's the best boss fight in the trilogy. The consistent leadup he has from literally the Prologue of the game is just screaming "this guy's a huge fucking threat."

He has so many phases and it's ridiculous. From the final Kaiju battle with Sin Gammorah to start the engine off is amazing. The second phase in space is awesome, the finale in the dreamscape or whatever, and then having both Bayonetta's 1 and 2 show up and LITERALLY BECOME ONE with Bayonetta 3 is just oozing fan service. Also can I just say Labolas looks so good in a 2022 graphics engine???

I'm probably in the same boat as a lot of people and was expecting a 2022 edition of Queen Sheba or Omne, but the fan service of having all 3 Bayonetta's fight and beat the shit outta Singularity with their Madama Butterflies together makes up for it.

The fight ends with an unfortunate note for Bayonetta 3, but having Gammorah deliver the true finishing blow is poetic justice. First demon on screen back in 2009, kills the final boss 13 years later.