I've only been doing it about a year so far but here's a list of my favourite Gundam kits that I've built, organized into the different scales/qualities:
HG (144:1)
1. Graham Aker's Brave Commander Test Type: 00 is my favourite Gundam series. All the suits in it are awesome as well as the characters. One of my favourite characters is Mr Bushido, Graham Aker. He's all about melee fighting and has probably the most unique Gundams out of any pilot ever. This suit is the one he used in Awakening of the Trailblazer.
2. Astray Gold Frame Amatsu Mina: I love the Astray side story, it tops my list of what story I'd like to see get an anime. This suit is easily the most non-conventional suit not only in the Astray line but in the whole Seed world. Lots of fun to build.
RG (144:1)
1. Justice Gundam: Even though I can't stand Athrun, his suits are the business. Justice has probably the coolest flight pack in the seed world and only 00 Raiser compares to it in all the Gundam worlds. RG kits are a lot of fun because they're so detailed at such a small size.
2. Gundam Mk II (Titans): The Mk II is a bit of a sleeper suit. Overshadowed in it's own series by both the Zeta and the Hyaku Shiki, it's a great suit that never really got a pilot that could use it to it's fullest. This is probably the best suit in the entire RG line and the colour scheme for the Titans version is just so cool.
MG (100:1)
1. Zeta Gundam (2.0): Only a few suits get updated versions simply because they're so popular. In Japan, the 3 most popular pilots are Char, Amuro and Camille and their main suits have all had at least 2 versions (Amuro's RX-78-2 has had 5 MG's alone and that doesn't count the variants based on it). The Zeta is of course a powerhouse suit that has been piloted by some of the best pilots in Gundam history (Camille, Amuro and Shin Matsunaga have all had custom Zeta's at some point). Zeta is wicked because it can transform, and do it well (unlike Bandai's awful Macross transformable kits) and it has a great arsenal of weapons highlighted by the massive Mega Launcher.
2. Ez-8: The 8th team is my favourite UC series and Shirou is my favourite UC character. This ground-based Gundam is actually pretty low on the list when it comes to specifications and abilities, but Shirou more than makes up for that with his sheer tenacity. Not a newtype like the other main pilots, he gets it done through hard work. His suit has a good range of weapons, a cool backpack to carry them all in and it just oozes 0079-era class.
