I can certainly understand why you're treading lightly here. Having recently been in more of the husband's position (only it was about sex, not romance), I'm going to give it from that perspective. I'm also assuming here that you and your best friend are much closer than you are with his wife. In other words, if they got divorced tomorrow, you would still be his friend but you probably wouldn't see her any more (this romantic issue aside, of course).
I personally would talk to the husband, your best friend, first. He's the one caught in the middle; if this goes south, he could lose his best friend and feel strain in his marriage. Make sure you tell him you're not expecting an answer right away, you just want to tell him how you're feeling and give him a chance to talk about it with his wife. He may then give you his approval and suggest you talk to her yourself; or he may tell you that he needs to see how she feels before he shares his opinion.
It's possible that for him, this will be a no-go. In that case, it's best not getting the wife's and your hopes up if it's not going to happen. In other words, it's easier to "turn it off" before it really "gets turned on."
Chances are, upon bringing it up with the husband, there will still be a need for a conversation between the three of you, whether or not anything is going forward. But my feeling is that if you talk to just one of them at a time, it doesn't put them "on the spot" in front of their spouse before they get a chance to talk privately as husband and wife.
Another option is to give them both a letter declaring your feelings, and giving them time to talk privately with you not there, and to get back to you when they've talked. That way you're not going behind anyone's back, but you're also not putting them on the spot to talk to you about it before they've had the chance to talk to each other...
Last edited by SchrodingersCat; 06-26-2012 at 05:28 AM.