Gonna give two answers to this, lucky you question asker!

In Mobile Suits that have shown up in anime it's hands down the Full Armor ZZ Gundam. Honorable Mention to the FAZZ as well but that's side story stuff so it only gets the HM. The Full Armor ZZ is just the dumbest looking truck ever built that barely even works when you think about it for a few seconds.

Non-anime I'm gonna cheat a little here but its nearly everything the Titans Test Team has in Advance of Zeta. Yeah some of them are normal but then later units fall directly into the "this looks so fucking stupid it rules." category.

oh oops this was all UC uhhhh, Wing Gundam Zero Kai for something non-UC but that feels like The Obvious Answer so gomen.