Major spoilers so don't read unless you have seen the series or you want to know what happens. Just skip down to the score.
This is without a doubt my favorite Gundam series. This also happens to have the saddest Gundam ending that I have ever seen. The story is set in the UC universe. The One Year War is close to ending, but not yet. On this one colony, a Federation base is testing a new mobile suit out for Amuro Ray, the protagonist of the First Gundam. Its the NT-1 Alex, and its damn powerful. So the Zeon sends a force in to take care of it before Christmas, or they'll just nuke the place and kill a whole bunch of civilians.
Bernie Wiseman is the main character, and he pilots a Zaku (I'm not an expert on the model and stuff like that so I'll just call it as I see it). Unlike a lot of other Gundam protagonists, he is not an ace nor does he have any Newtype abilities. He's just a new recruit and this is his first mission. The other team members, though, are experienced pilots and they have the newest mobile suits to be made. A Rick Dom, Gelgoog, Kampfer, and Hygogg. All Bernie has is a Zaku.
So they arrive on the colony to destroy the Gundam. On the colony, a grade schooler named Alfred is obsessed with war and knows more about the model numbers than I do. This guy discovers the Zaku after a brief skirmish between Zeon and the Federation. Bernie lets Alfred live, and they become fast friends.
That is where the heart of the show is: making everyone human. Bernie wants to seems cool so he lies that he is an ace, but when they go to the Zeon hideout, the others make fun of Bernie and says that he is a new recruit. Also, Alfred's friend, Chris (a girl), meets Bernie, and its obvious that they like each other. And that's also a reason why the ending is so sad.
You see, Chris is the test pilot for the NT-1 Alex. When the Zeonic force tries to attack again, Chris kills everyone. Bernie was not there. And so, the final days come, and on Christmas Eve, Bernie makes plans to destroy the NT-1 Alex so that Alfred's colony will not be wiped out. In a magnificent episode, Bernie does what everyone else together could not; he destroys the Alex. Knowing that he cannot win in a direct assault, he lured the Alex into the mountains where traps were set.
And thus, the sad scene occurs. The Alex impales the Zaku with a beam saber; The Zaku beheads the Alex with a heated axe. Bernie dies, while Chris is injured and transferred to Earth. But shortly before the fight even occurred, Alfred learned that the ship that was gonna nuke the colony was captured. There was no need to destroy the Alex anymore. All that was for nothing. At the end of the episode, Chris asks Alfred to say goodbye to Bernie for her.