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.
Show
It's not techinically a "gundam", but damn do I love the Psycho Zaku from Gundam Thunderbolt
-Mobile Suit Engineers
My second pick is Wing Zero, because it turns into a fighter jet, even though this functionality is rarely used in the actual show.