Tradeoffs and design considerations when choosing a crystal + PLL combo for MCU timing
Having got the datasheet, I'll now give a more general answer as to how I'd approach it:

Firstly, do you need a precise frequency in the first place? There is a certain amount of slack in most applications and you can probably just use one of the internal oscillators. I see this part provides a 32khz RTC oscillator and a 48MHz (USB2) one.

Secondly, is there something that does need a precise frequency that I can use as an input? For example, PAL video or USB. If I have to use a crystal, I don't want to have to add another crystal to the BOM if possible.

If I've got a free choice, pick a crystal in the middle of the operating range of the PLL which is cheap.

Smaller divider values are better for jitter, but check your fractional PLL's available fractions.

For your specific application of a UART, it's common to oversample because there may be jitter on the other end. So you might want a sampling frequency x3 or more times line speed.