I know this has probably been done before but I searched and didn't find anything (no reason is necessary)
Faith
Tara and Willow (I think most fans of the show know why)
Buffy
and
Spike

I'd go with Buffy and Anya. After last season's crap with Willow, I can't stand Willow anymore.

On Buffy--

Spike and Anya were the only characters I could stand after season 6, although I'm liking everyone else again this season.

The easy choice for me though is Wes as his character has evolved on Angel.

tasha

(favorite villain is the mayor)

willow from seasons 1-3 was awesome
Xander seasons 1-5
Angel
Anya
Dawn season 7
Cordelia
Oz


least favorite

Buffy season 6-7
Kennedy (she needs to die ASAP)
Tara

Anya, Faith and Angelus

Season 1-3 Willow
Season 4-6 Anya
Season 7-? Andrew

I mainly had a preference for the characters because of how funny they were. I think Willow hasn't been as funny the last couple years because Anya was getting so many of the jokes. Now Andrew is getting all the best lines. Andrew is actually making Xander funny too. I so hated Xander being forced into bad comic relief in season 2.

anya and cordelia. 2 of the funnier characters.
faith.
and oz. don't know why he left but he was awesome.

Spike and Willow

My favorite is/was Giles. I never liked Xander at all. Dawn is not bad now, since she is out of her rotten punk kid stage, and Willow is okay. I like Spike and Buffy, and Anya is okay as well. But overall, I always liked Giles (and I loved that they brought back his cue card artwork this season (that he first showed in Hush) for the girl that couldn't speak any English).

Giles rules!

I'd say Dawn, Xander, and Spike.

Season 1 - Giles (I guess). He was the least annoying character of that awful season.

Seasons 2-3 - Giles

Seasons 4-5 - Tara and Giles

Seasons 5-6 - Tara and Anya

Season 7 - Don't know yet.

Oz. I can't get enough of Seth Green. And I say that in a purely heterosexual way. How I wish he'd join the gang on Angel.

Buffy

From start to finish... it was always Willow that had me watching the show

Man, I must be the biggest geek here. I can't believe nobody else has mentioned Andrew. So far he's definitely the best thing about the show this season and I didn't even like the trio last season.

Spike, Xander, Anya and Cordelia. They're about the only characters that haven't said/done something to make me roll my eyes, at least not in a serious way.

Oz. For being both hilarious and a perfect example of how a secondary character's SO can be developed and relevant without sucking up screentime.

Honorable mention goes to Jonathan, pre-S6, and S1-S3 Willow, back when it was okay to be a nerd.

Xander, Spike, Giles, and Faith

The Mayor

A close 2nd would be Xander before he got fat(i.e. through the end of Season 5).

Xander and Spike... with Andrew as the best newer character.

Oz is a good choice, a very solid character whose time was limited there. I kept hoping for them to broaden that character, not get rid of it.

I'd go with Xander.

I also like Oz, Cordelia, Andrew but their stints have been brief.

Season 1: Buffy and Xander
Season 2: Angel/Angelus, Spike, Buffy
Season 3: Faith, Oz, Xander
Season 4: Willow, Tara
Season 5: Glory, Spike
Season 6: Andrew, Spike, Willow
Season 7: Andrew

My favorite regular characters are:
Spike
Willow
Anya.

My favorite single season character by far is "The Mayor."

I'm WAY late on this subject, as I have just finished season 2, never seen season 1 (plan on skipping it altogether) and am just starting season 3 (all on DVD - not watching the show until I catch up (which is never, right)). I would have to say my favorite from season 2 is Xander and then Oz (although he was barely on season 2).

One other comment...Xander hasn't really aged that well, has he? I watched the new show last night for about 3 minutes, and he and Buffy both are looking pretty bad compared to seasons 2 and 3.