You know this one is always the most difficult question I get asked about anime. But if I were to committ to an answer it would be Rei Ayanami. Here semi mystical existence and abilities set up a background of intrigue. She is a reincarnted clone of Shinji's mother but with the soul of an angel. It is very misterious. Plus the queit thought provoking Waifs get to my heart eachtime.
I also love Rei's poem. It has inspired many of my own works. Ow, and she is beautiful.