If I had to choose one modern line to focus on, I guess it would be the 3-3/4" line. They are the most economical for me to collect. They're also essentially all I collect that is Star Wars related now, other than autographs. And since I mostly have 3-3/4" carded figures autographed, it's a no-brainer.
On the other hand, if I had to focus my collecting on only one aspect of all Star Wars collectibles EVER, it would be rarer, higher-end and more esoteric items from the '70s and '80s, like the Sigma ceramic pieces, a ste of original, mint condition metal lunchboxes, etc.
And finally, if I had to choose only one thing, out of everything I collect, I'd probably have to go with original comic art & convention sketches. I love collecting mass-produced memorabilia, but there is a special feeling that goes along with buying a long-sought-after, one-of-a-kind piece of art, or a sketch/painting that was done just for you.