My vote is for Titan
The world has too many fascinating things we are just
barely beginning to learn about. We need to do more
than one probe sitting on its surface for a few hours
taking a few not so hot photos.
I like Europa too, but since it seems we have to choose....
And let's face it, digging through all that ice may be too
much for one probe. Of course there may be ocean material
already on its surface.