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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Not a gundam but I always liked the Grunts like the ZakuII
Zaku II is my favorite non gundam ms too.
Gundam has so many great grunts, the Zaku II being such an iconic design.
I particularly love the Grimoire from G-Reco:
And the Tieren from 00
I'm not sure if it still counts as a grunt, but Chuchu's Demi Trainer is awesome:
you comrade have great taste in mobile suit