Red_Avatar: Overseer was definitely different. It was basically a remake of Martian Memorandum but the characters were all different, the locations were all different and I think that's partly why I think it's the weakest of the three FMV games.
Pandora's Directive definitely is the best game of the three (technically as well as its puzzles) although UAKM is my favorite because of the general feeling of the game. They made a few mistakes there which they learned from but generally, it was an utterly amazing game for its time. Other FMV games looked horrible, had even worse gameplay and static pre-rendered backdrops and UAKM offered full 3D worlds, crisp FMV and a very slick interface. It's surprising to see how no FMV games except for Gabriel Knight 2 even got close and even Gabriel Knight 2 was leagues behind.
I think that Overseer was the weakest because it's the most 'ordinary.' They definitely tried to take advantage of their new engine / movement / looking system by filling every little ledge, nook and cranny with relevant items. UAKM is also a little bit guilty of this. PD has the least of "look at something from every angle to find the item."
Personally, PD for me is the best in the series and truly one of the greatest games ever made. In fact, it was the first game that my parents bought for me, right upon its release and I've been a raving fan since. In fact, I joined GOG as soon as I realised that they sold the series. UAKM is also very good (and I loved it as a kid) but it's a bit too campy at times. PD has a more serious overtone without sacrificing the humour element.