A lot of people are basing their choices on the appearance of said character, so I'm going to interject for a moment...
I've dated enough women to know there's only one thing you really need to be on the lookout for, and that's baggage. Otherwise, you're not going to have a good time.
Sylvanas Windrunner: While probably a freak in the sack, she has a whole lot of anger towards previous men she's been associated with. Dating her may be fun for a while, but the aftermath of the relationship ending may be a catastrophic.
Jaina Proudmoore: Extremely attractive, the kind of girl you'd want to bring home to mom, but there may be something off about this girl (even before Theramore got wrecked). Did you notice something about the men she dates? Arthas, Prince of Lorderon, pretty cool guy and would have probably made a great leader. Look what happened there. Kael'thas, went nuts after dating Jaina. Now she's dating Kalecgos, a Blue Dragon, and I'm guessing we can all see where this is going.
Alexstrasza: Personally, this would be my choice based on hottness, but talk about baggage... Alexstrasza seems more like the kind of lady to sooner bake you a pie than have a shag, but don't let that fool you. Alexstrasza is a loose lady, but not in the good way. Deathwing forced her to mate with two elder dragons, to which he took the offspring and turned them into his evil minions of burning doom. This chick popped out COUNTLESS offspring, being forced to hook up with two guys that she had no interest in. Would you blame Alexstrasza if she weren't into getting down with the guy she's dating?
Honestly, my choice would be Ysera.