There's not a definite answer, as you have not specified a seat configuration, but I will assume an all-economy aircraft such as the following British Airways configuration.

[![UK Domestic - British Airways][1]][1]

The triangular marker on this aircraft configuration appears above row 9. However, this does not denote the best view for the passenger. The flight attendants are standing in the aisle, forward of the window, looking down the leading edge of the wing. I've drawn their actual line of sight below:

[![A320-LOS][2]][2]

Obviously, the passenger does not have the best view as he would have to crane his neck backwards to see the wing. The best seats for a passenger would be the ones where you have a view of the wing, so window seats rows 15-18 would serve you best. Personally, I opt for the emergency row seats every time - they give plenty of leg room, aren't much more expensive, and provide a pretty good view of the top of the wing.


  [1]: https://i.stack.imgur.com/jCYJj.jpg
  [2]: https://i.stack.imgur.com/E6cg8.png