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My first Lego was back in 1976 (cripes!) - 404-1: Universal Building Set, which I don't actually remember getting as I was only three. The first set I actually remember getting was 6950-1: Mobile Rocket Transport for Christmas in 1982. Very, very fortunately, I actually still have all the sets I was given as a kid and even most of the instructions. I think my Lego was one of the few things my mum didn't decide I no longer wanted, though it did sit in cardboard boxes for a few years until I seriously got back into building and collecting in 2003.Cobweb
These day I tend to favour Star Wars sets, Modular Buildings, and anything with lots of interesting and usable parts, even themes like Friends and Elves as there are often good pieces in fun colours. As I primarily build MOCs (the bigger the better!), most sets tend to get built once, when I get them, then the parts are incorporated into the collection ready for use in whatever project is on the go or in the planning stages.
I used to build mostly space stuff as a kid (being a huge Star Wars nerd right from seeing IV at the pictures, never houses or "normal" stuff as I never really had enough plain bricks to do stuff like that. Then I got 10190-1 - Market Street because of the colours and quantity of parts and suddenly discovered the joy of building elaborate buildings with full interiors and so on. So now I make sure to get every new building that comes out. They still generally tend to be built only once but I'm now very definitely into designing my own, from medieval through to futuristic, plus landscapes.