Agree with Bryan, ultimately you should choose something you connect to and beyond that go for something broad and versatile. By versatile I mean a character that can also be associated with certain vehicles or play-sets if you wanted to ever extend your focus.
I chose Luke X-Wing Pilot because I remember being chuffed as a kid when I could turf Luke Farmboy out of the cockpit of my Pali X Wing fighter and replace him with his orange-uniformed murdering bastard other self
X-Wing and Luke pilot - for me it's hard to separate the two as to which I like best - so I collect both
My small and humble focus has taken ages to grow (a good thing I think!) and I always shift between adding a new figure or a ship or even related oddball stuff. This, for me, always keeps the focus fresh and interesting.
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imperialgunnery.com Vintage Star Wars action figure weapons and accessories guide