I would suggest that the original Half-Life had one of the worst endings of all time. Then, if you played Unreal Tournament III, that's got an awful ending as well. Then, I would put Sins. Above that, I would put the ending to Windows Solitaire.
A problem I have with Sins isn't so much the lacklustre ending screen, but the endgame in general. There seems to come a point in mid-game where I know for sure me and/or my allies are going to win or lose. After that point, little seems to affect the outcome of the game. In the games I have played, there is not much tension to the end game. Even in Civilization, if you know you are beaten, you can still hang on and make your mark in the history books. Sins ending to me is more like binary: 1=faultless victory, 0=humble defeat, with little in between.
Of course, your mileage may vary, and I am only speaking of single player game play. Still, Sins is definitely cooling off on my hard drive. At least it wasn't expensive to buy.