Do you spend the same kind of quality time with your husband? Does your husband make you feel appreciated, attractive, and womanly? It sounds like he respects and loves you and has complete faith in you.
A harmless small crush is not a big deal- something natural, like you said, such as fantasizing about something that you can't have.
But if you think you might drunkenly do something more with this man, even kiss him then you may be treading on thin ice.
As much as you enjoy the time you spend with him, you should be recieving those kinds of feelings from your husband and not just his best friend. Even if your husband is a busy man, make sure you have good conversations with him, make sure that you both are making the other feel wanted, appreciated, and loved for who you truly are.
Other than that, it may be a good idea to find some more female friends, learn to rely less on sensitivity from male friends, and let your husband know that you want to be feeling those things about him.
You can think whatever you want about his friend, have your fanatsies, but don't tempt yourself by spending so much time with this man and then particularly discussing how wonderful he is and how crazy it seems that he cannot find someone to date.
If you must spend time with him, maybe the majority if not all of the time you spend with him, should be with your husband present as well. if he is truly your husbands best friend, you and your husband have a good relationship, and you are obviously comfortable with this guy, then it should not be uncommon that the three of you talk about these things together, instead of the two of you.
Also, it wouldn't help if you all also spent your time out together(such as nights when a d.d. is required), with other mutual friends so that he can focus some of his relationship talk towards them, not just you.