Definitely not my first MOC (I think that was Han Hoth), but my first I really have a clear memory of opening as a child was 4-LOM, can literally still remember the smell (is that weird or do any others remember 'that new SW smell?) of busting him open, even all these years later.
First MOC collected as a proper grown-up was Lando Skiff, bought with a lightly battered card and lifting blister; can't believe its stayed attached all these years. If I ever get an upgrade, I wouldn't sell him though.
Still have both today
... sadly only 4-LOM and a Biker Scout survived my teenage car-boot cull of my childhood collection

First MOC collected as a proper grown-up was Lando Skiff, bought with a lightly battered card and lifting blister; can't believe its stayed attached all these years. If I ever get an upgrade, I wouldn't sell him though.
Still have both today