PortAventura only has / used to have a front row queue available for Furius Baco. In fact, Dragon Khan has some cattlepen in the station that I think was meant to be used for front row queue as well but PortAventura decided not to use it.
As for the rest, you are ALWAYS assigned where to sit, with no choice at all. You may ask the attendant but as they need to get the line moving you will probably have to sit where they tell you to.
And single rider, only on Hurakan Condor, but that single rider is just a joke. This concept has never been used in Spain before so it's not well organised. In fact, the single rider queue starts right in the middle of the cattlepen, so you still have to wait with groups for the most part even if you are on your own. And then you see lots of groups joining the single rider line thinking it's a quicker way to get on the ride and when they're told to split up they get mad at the attendant.