Bb
The Bflat shape is very important as it is one of the basic moveable ones. Slide it one fret higher, and you are playing 'B' - 4322 - one more gives 'C' - 5433 - etc. So you must master it. It is easier to manage if you place your thumb at the back of the neck. Some chords are OK to play with the neck resting in the web at the base of your thumb, but others require more mobility of the fingers. When I play Bb my thumb is placed at the back of the headstock, between the nut and the first two tuners.
I remember it being difficult when I was learning, along with Dm7 (2213), F7 (2313), and of course our old friend E is a swine!
Practice, practice, practice. In a short while you will wonder what the problem was.
Ukantor