Apart from the obvious choice - the back and front row - I'm never usually too fussed about whereabouts on a train or on a row I sit.
The only exception is Nemesis. Regardless of the row, I love the far right hand seats (as you look up the train). The only reason is because as you're thrown over the station in the Zero-G roll, you can really reach out to your left hand side and make out as if you're going to touch the station. This really disorientates me, I love it. :D
But apart from that, meh, wherever really.