Anything with a clear display and three hands.
A stopwatch is nice to have, but not particularly essential at PPL level.
I like a velcro strap, then it can go around the middle of the control column, but that's just me, and certainly not essential. I have an "Animal" leather and velcro strap from an outdoors shop - it gets replaced about every 3-4 years.
I do use a reasonably posh Seiko Military that I'm very fond of, but I didn't pay for it, it's 18 years old and I wouldn't necessarily pay for an identical replacement if it died. There are plenty of perfectly good watches out there which would do everything mine does, for around a tenner.
G
Postscript, my Seiko is a lot cheaper than it used to be - £99 on Amazon, and I am fond of it, so I probably would. However, the one on my wrist is still good as new (okay, one new glass, and probably 4 batteries and 5 straps in 18 years) so I shouldn't need to. This one.
- or near as dammit anyhow.