Wes and Lilah.
I always like messy relationships better than cutesy ones. Really, I liked both pairings. I love both Fred and Lilah. They're both awesome characters. Wes respected them both in different ways. But Lilah felt stronger. I know they both died, so neither one ended up that strong. And both deaths were tragic. But between the two, Lilah was the character who had more substance. She felt solid and real and tangible.
Here's the thing: Wes put Fred on a pedestal. She represented an ideal. Wesley loved and respected her goodness and her warmth. Eventually, she would have fallen from that pedestal or gotten tired of being placed on it because, as we know, she isn't just good and kind and warm. She's a complex individual, just like anyone else. With Lilah, it was different. Wes hated her at the start, but he grew to respect her for her ambition and her ruthlessness and her strength and her cunning. Her strength made him stronger. He became more confident. He began his own outfit. He saved Angel. He really became his own man.
Both relationships seemed doomed from the outset. But, to me, the relationship with Lilah seemed more beneficial to Wes in the long-run and, also, more real and exciting and true-to-life.