I didn't see the other Gundams, but I really enjoyed Iron-Blooded Orphans.
Well, Iron blooded orphans simply easier to understand for non Gundam fans that doesn't really understand the lore.
Because those amateur pilots also didn't know what the heck they just discovered at the ruins.
Well, most Gundam in the past are just like that.
Filed with sad tragic background, allies dying, biased loyalist enemy pilot, more tragedy, finally saw enemy doing human experimentation, more allies unexpectedly dying, more sad and depressed MC, and there's no certainty whether the MC will survive or Not at the end of the show.
It's either bittersweet fight, or bad ending where MC doesn't survive, or the rare happy ending.
MC usually wins if they're careful, since they've become powerful over the time, albeit with lots of sadness. But if the MC is just too reckless as Iron Blooded orphans, or just facing too strong opponent, they may die and the series will end.
Because the Gundam series is really about the MC and his Gundam until they became separated and no longer fight.
----------------
Well, even in the Gundam Unicorn where it shows happy ending, it only having happy ending and the next series to satisfy the fans. It's because of success and fans that it has good ending.
In the novel source, it's unclear whether MC survive or not in the first story. It's just unclear what happened after MC talk to his dead parent using new type magic or whatever.
I bet MC probably didn't survive due to aftereffects of using too much power and ends up drifting in space and died with his Gundam. And ironically meeting with his died parent that told him to survive.
------------------
And the newest Gundam Hathaway,
It's gonna be bad ending.
It's kinda unexpected for the 30 year old novel to be adapted into anime. But it pave the ways for true tragic bad ending = MC being executed by his own parent.
-------------------
Sorry for long comment.