In the archives this subject is pretty well covered.
It seems an equal spilt between water and oil based primers.
My inclination would be to opt for oil based.
We all know MDF swells after water contact, so for that reason I'd not go
for water based.
This is to prime the bit of mdf that I've exposed by routing another profile
over an old one, on some skirting that'll eventually get white gloss.

--
Mike W