Whatever fits best.  Spend good money on an undersuit.

Try before you buy.

If you've got a lot of money then get a made to measure, if not get a low
cost tri-laminate (membrane) suit that fits well and don't worry if you need
to change it after a few years.

Fit and comfort are the most important things (apart form staying dry).

Spend good money on the undersuit, and once again, make sure it is neither
too baggy, as material folds will hurt if you have suit compression (often
called squeeze), however its even more important that it does not restrict
your movement in any way.

TonyH