I choose Gendum Exia,OO Gendum and OO Quanta.
What's yours one?
IRL reason: Because Japanese fans complained the show wasn't "Gundam enough". Thankfully they fixed that with Quanta.
In-Universe reason: The S1 and movie designs were made in cooperation with Veda, whereas the S2 designs were made separately by Ian and Linda's team.
Funny how back in S1 not even the Thrones had the face vents, yet in S2 only the red Gundams that lacked the vents.