In my opinion, Blackadder Goes Forth had the most depressing ending of all. Considering the entire rest of the series had been a funny, snarky comedy, the ending to the very last season takes a very depressing turn.
In the last episode, it is finally time for the final push over the top and and out of the trenches. Captain Blackadder desperately tries to get out, only to have his last attempts end in failure, Captain Darling is sent to the front by General Melchett as a "gift", when he wants nothing more than to stay OUT of the trenches, and Lieutenant George is very gung-ho about it all... until about five minutes before they are all going over the top, when he suddenly realizes that he too is scared by the prospect of dying. The last we ever see of them is when they charge out of the trenches, into the enemy's gunfire, with the theme slowly and melancholy playing in the background, before the whole scene dissolves into a field of poppies. No credits whatsoever, The End.