Take a look at TGs seat maps here: http://www.thaiair.com/flying/seatmaps.htm

The exit row upstairs - row 16 or row 17 depending on the 747 configuration - is difficult to get. TG usually assigns those seats on the day of departure, so try to check-in earlier rather than later.

Be aware also that the first row upstairs is near the lavatories and can sometimes, but not often, be smelly. It is a high traffic area too. Also, the pull-down TV screen is very close, making it difficult to watch the movie.

The row at the rear is close to the galley, which can be noisy and bright, especally if you are a light sleeper.

If you can't get the exit row, I'd suggest you go for the row behind the exit row, or the row in front of the exit row, but it all comes down to personal preferences.

Good luck.