I went with Willow/Tara, as it seems most other people did. They just really fit well together as a couple and brought out the best in each other - at least, until Willow went Dark Side..
Willow/Oz would have to be my second favourite and I thought it was v. cute in Oz's first few episodes that he spotting Willow and wondering 'Who IS that girl?' in a way that no one had (but, in my opinion, should have as Willow is such a sweetie) before. Plus, I loved their little conversations like, not sure if this is exactly right but, 'I'm thinking of asking you out' 'Well, if it helps, I'm going to say Yes' 'Well, it does help, you see as it creates a Safety Net'... etc.
I liked Willow/Xander for approximately one episode, S2E1 'When she was bad' - the cute little ice cream on nose incident which was rudely interrupted by a Vamp.. But the cheating/fling/romance thingy in Season 3 really didn't say a lot for either character in my eyes..
I'm afraid I have never cared for, or to comment on, Kennedy.