For me it is the Shining Gundam. When I first tried out SDGO when it first opened in Korea I saw that in the shop. I did not have enough time to play back then so I asked a Korean friend to purchase some credits for me using his card and I bought enough points to build it.
It is still my favourite unit until now, but I'm afraid my lag doesn't allow me to enjoy playing rocks very often.
In SEA I have the quest one (and I made the Shining BM) but I will be making a new Shining Gundam from plans so that I will have a C3 one at least (and the quest one will probably be eaten by God Gundam assuming Cherry Credits doesn't release it as a "Top 3 Biggest Spenders" only MS).