Temple of Seti I, Abydos
Constructed in fine white marble by Seti I (1318-1304 BC). Cutouts in the roof allow blocks of sunlight to light up the interior of the Second Hypostle Hall.
Abydos is a little bit out of the way from the normal touristy route, so you won't see as many visitors here as you usually do at other temples. It takes about 3 hours by car one way to reach here from Luxor, but I think it's a very worthy temple to visit. It's well-known for the exquisite bas-relief carvings on its walls, and the intact roof and lower height of the columns in the hypostle hall give it an intimacy that I think other more well-visited temples lack.