Window seats always, but exactly what I'm flying on and how long of a flight are the determining factors. For example, on a 737-200, I will only sit towards the front of the plane, period - even hush-kitted, those P&W's are just too loud in the middle and rear of the plane! For 737-300's & above and A319-320's, short hops I'll sit anywhere, but anything over 2.5 hrs I'll try to get up fron if possible. I've never flown on a 757 where I've been seated after the wings, but in walking around inflight in the back of them, they seem to be pretty quiet (AA SJC-BOS 757 redeye a month ago). Same goes for 767's. On DC-9's, MD-80's and 727's, I try to get up front if I can - the front parts of the -80's in particular are amazingly quiet! However, the same rules apply - longer flight, the closer to the front I prefer (although everyone should sit in the rear of a 727 once in their life to see their wings in action - cool flaps, droops and spoilers!). I've flown the DC-10 once and got stuck in the middle of the 2-5-2 arrangement in the middle of the plane - these had to be the worst seats I've ever had on a plane. On RJ's, I have no preferences, since it's very difficult to see out those windows anyways and the cabin is pretty quiet no matter where you sit.
Propeller-driven planes - window seats in the rear always! You get to see the wing, propeller/engine combo, it's quieter, and sometimes you can see the landing gear in it's full operation (extending, retracting) and sometimes you can see the wheels actually touch down - very cool!!! (Dash-8's and Beech 1900's come to mind!)
One other reason I like being closer to the front is that I'm a pretty frequent flyer who often times has to run off the plane and through a crowded terminal to catch my connecting flight!! On the commuters, I can sit in the back since it usually does not take too long to disembark.