If I were given the choice, I'd choose Budapest.
It is central, growing and easy to commute to and from if necessary.
Warsaw is a nice base. The city is OK too. I enjoyed my occasional flights from WAW.
Belgrade is also a nice city, but it is the smallest of those bases mentioned. Therefore, I'd aim for the biggest one first as you'll have more variety and very likely more sectors, which in Wizz can mean more pay.
Those 3 bases are by far from being the "worst" bases in Wizz Air.
You'll enjoy any of them.