Any domestic positioning flight on a internationally configured widebody in FC is my favorite. 747, 777, 767, doesn't really matter to me.
For "US only", Seat 1B in the old UA DC 10 was my favorite domestic seat. Tons of leg room, no one reclining in front of you, and acres of overhead bag space. Given the cramped 757s and A320's that's replaced it, really makes me miss that plane.