It's easy enough to pick any of features listed, but I have to go with what Chris said. Its a bit of a cop-out, not selecting any single feature in favor of all of them, but it's the most honest answer I can give. I usually find depending on the scene in an episode or film, or which photo of Summer Glau you are looking at, your eyes are generally drawn to one or more of her features.
Her smile. Summer obviously is a very pretty woman, but when I met her it was her dazzling smile that was the first thing I noticed, and that I kept being drawn back to. In photos I would probably say her eyes, beautiful brown liquid pools, but in person it is her smile.
Nice thread picked by RedsBaron. He is lucky meeting her in person. After reading the oppinions of others, I'll try to answer myself into a creative kind of way: I would go with something like what Wash said in firefly "Bushwacked" for his wife, starting from Summer's eyes.. until she smiles! . To the Sweetest Terminator in the Verse..., Summtris (game), Twitter