In my opnion, Tumble of course.
First, If you are familar with the dungeon, you will know all the spots of hidden doors.
Second, If you got the sand's goggles or Tarne's goggles, true seeing helps you to see all hiden doors and hidden monsters. (I didn't put any skill in spot , can still can see most of the invisible mobs, maybe because of the true seeing of the monk's high wis mod).
Third, tumble for backflip(31 or higher) and forwardflip(36 or higher) is cool.