Great thoughts. I'm a slasher, but I'm mostly a Xander slasher, so I don't notice or care to notice other characters' being slashed. However, I do find Angel more enjoyable to watch when he's interacting with characters who are not Buffy. I like his interactions with Giles, Xander and Willow, and find that he becomes more at ease and himself when he's around them. Xander and Angel have awesome interactions in Prophecy Girl. Angel and Willow have great scenes in Lie to Me.

I definitely prefer later seasons Xander.

As a huge Xander fan, I like him in all seasons. Preferring him in high school years or later years depends on my mood. I find him more interesting in the high school years because he's more complicated, and like his maturity in the later seasons. Right now, I find myself enjoying him more in the high school years, I love flawed characters. But when I read fanfics, I prefer reading about him in the later seasons since fanfic tends to use him more the show.