Mine is pic related, the Victory Gundam. What I like about it is how modular the mobile suit is. It's the result of years of ms tech late into the Universal Century where now the Gundam is mass produced. In the show the rebel forces are shown to have a stockpile of Gundams in a bunker so they can burn through parts.
Honorable mention to the RX-79 G ground type Gundam from 8th MS Team.
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I'm relatively new to Gundam compared to a lot of people here maybe, but right now I think the Schwarzette is a pretty cool design:
Also, maybe this is heresy talking about it here, but some of the recent Transformers designs are also pretty neat:
Can you tell that I really like glowing neon swords?
schwarzette rules
also if we're talking transformers i like the valkyrie from macross (which was stolen a few years later for transformers)
That is a pretty cool design. Also, I wasn't previously aware that Transformers took anything directly from Macross, but I just looked this up and discovered that the team who created the original Transformers mech designs included Shoji Kawamori, the creator of Macross, which is pretty interesting.
EDIT: Kawamori was just part of a larger team, and other other designers also had a hand in creating some of the more well known Transformers characters. Interesting video on that here.
the valkyrie turned into the transformer jetfire! it was also stolen for some battletech mechs, as were a bunch of mechs from fang of the sun dougram and crusher joe! the rights to macross are a mess