I would say I find FOR YOUR EYES ONLY, LICENCE TO KILL, TOMORROW NEVER DIES, and QUANTUM OF SOLACE to all be pretty underrated.

For Your Eyes Only is pretty easily the most underrated film in the series, in my mind. It used to be On Her Majesty's Secret Service because people dismissed it due to George Lazenby playing Bond in it, but in recent years the audience has turned around on that one and it now routinely finds itself somewhere near the top of the list when it comes to great Bond films. For Your Eyes Only gets largely ignored for being very low key and for the fact that it's Roger Moore, at the time already too old for the role by a long shot, and it came right after Moonraker and right between Octopussy. Two very low spots of the series.

Moonraker, whose script is far less silly than some claim it is (and has far less plot holes than 'Skyfall' for instance).

For Your Eyes Only.

Easily.

It's my most favorite Bond movie, even if the incidental music is "dated" half the time... the gripping, taut direction and engaging plot make it a WIN. And Sir Roger Moore getting revenge on a creep that killed, amongst others, a person he genuinely fell in love with (which happened only once before...)