David Boreanaz, they always had the BEST chemistry. And I believe they even had a relationship together in the past. David Boreanaz is very caring of Sarah Michelle Gellar. He said, when playing Angelus it would hurt him to make Sarah cry, and afterwards he always made it a point to give her a hug when the scene was done. Also, someone on the set was being mean to Sarah and James Marsters said David wanted to kill that person that wasn't treating her nicely.
Oooh, and Sarah says David is definitely the best kisser out of all the guys she has kissed on-screen, which includes James Marsters. Go David! (He looks like a wonderful kisser, she's so lucky).
Don't get me wrong though, I find Freddie/Sarah very cute together. But, if they weren't together, I'd think her and David Boreanaz would be a match made in heaven.