Two words: "personal preference".

You can run 28's or 32's or 35's or even 45's on most hybrid frames and forks.
Some people like to run a wider tyre in the back and a narrower in the front.
For both roads and moderate offroad I highly recommend 35 cyclocross tyres.
I use Schwalbe CX comp's which are cheap yet robust and fast ... there are several others in various qualities and prices you could consider ... Marathon supreme's are highly recommended aswell.

Be wary of running anything smaller than 25's though as you may need a narrower rim for those.