I think @Thunderforge Nails it, but here are some runner ups:
Note: times are estimates, but should be pretty accurate. Also, I'm not including mentions in the appearances.
Aurra Sing: 28 Appearances / 2 second = 14
Depa Billaba: 37 appearances /
3 seconds = 12.3 8 seconds = 4.6
EDIT: you see her in The phantom menace, but the back of her head also makes an appearance in Attack of the Clones, which reduces her total score
Dengar: 49 appearances / 10 seconds = 4.9 While not an impressive ratio, he is only present for a single shot of him in the distance. We don't even see his face!
When I first read about him, I thought I had found a winner,
Davin Felth is the stormtrooper who says "Look sir, droids!" in ANH, unfortunately, this site prooves that he's got a lot more than half a second of screen time! He does make 12 other appearances though.
Ewoks: Ewoks ran for about 35 episodes and included a number of ewoks that appeared in ROTJ, however, from what I can tell, most ewoks have at most about 45 appearences outside of them films and at least 20 seconds on film, which doesn't bode well for them.
Cantina Patrons: A few cantina patrons have pretty fleshed out backstories, unfortunately, due to the long panning shots and reused characters in this scene, and the fact that only a couple make more than a few appearances: only one stands out: Kardue Malloc has 1.5 seconds of screen time and makes 14 non film appearances. 9.3 is his ratio.
Stormtroopers Only a few stormtroopers have more than a couple non film appearances, making this one a tough catagory.
Rebel troops Not too sure on this one, but I do know that a lot of extras were reused in multiple scenes, so one character might have a lot of screen time.