Kind of interesting that american is even listed. There are a number of accents, but strangely I don't think ONE of them is really considered "attractive". There's Southern accents (Howdy Y'all!), California/Valley Accent (what-EVERRRR!), New York Accent (Fuhgeddaboudit!), Bostonian (TF2 Scout)... and those are just the more famous ones. There's "yoopers", (upper peninsula of Michigan. Close to stereotypical Canadian), Ebonic (Every rap singer EVER), Arkansas (Bill Clinton had this). The only american accent that could even be considered remotely attractive would be an upper-class harvard/new england accent, but that's because its just british with flatter tones. Worse yet, the only noteworthy example of someone who USES that accent is none other than Stewie Griffin* from Family Guy.
As for my vote, it gets me going to hear a female voice with any eastern european accent, so russian would be my favorite.
*apparently the accent was taken from actor Rex Harrington.