There hasn't been a character popularity poll for some time now. The last poll was a "best battle" poll and the Rukia Kuchiki vs. Aaroniero Arruruerie came in 7th. Rukia consistently placed second in all four character polls conducted to date (with Ichigo coming 1st in the first three polls and Hitsugaya coming 1st in the 4th character poll). Rukia's Sode no Shirayuki also came 2nd in the Zanpakuto Popularity poll (with Hitsugaya's Hyōrinmaru coming first). So on the basis of popularity polls - it would have to be Rukia.