Maybe not my best seatmate, but surely one of the most memorable: first class, Delta, DFW-ATL, bulkhead row. Tall, very well-built young man (I'm a straight male, BTW). We got to talking about my computer - one of the early portables. He said he could use it for his part-time master's program as he travels a lot. I said it was great but its 16-pound weight might be a problem. He said he didn't think it would be for him. We kept talking and a bit later on exchanged business cards. It was Dave Johnson, then training for the 1992 US Olympic decathlon team. He _throws_ 16 pounds further than I could _carry_ them!
Best was probably returning from a long weekend on Grand Cayman with my girlfriend. It was a spur-of-the moment thing to escape from a New England winter. We flew down (DL) in coach and she didn't know I had booked the return in first. When I started for the door as they pre-boarded first she asked "shouldn't we wait for them to board Gold Medallion?" I said "no" and showed her the boarding passes (carefully kept hidden until that point). She was all smiles. Stretched the vacation a few more hours, more so than if we had been in first both ways.
Worst: BOS-ORD, UA coach, Met Life insurance salesman who insisted on going through his entire PowerPoint presentation for me. Packed flight, I had the window and he was in the middle, so there was no escape.