Okay, a couple of ground rules:
1. These are a matter of opinion, not fact.
2. Try to start from the assumption that someone's dislike of a character you love is a reflection of their dislike of the character and not an assault on you, personally.
3. Don't resort to ad hominem attacks while discussing the merits (or lack thereof) of characters with your fellow forumites. They may be fictional characters; the rest of us are still (last I checked) humans, so keep it cool.
4. Optional, but think on this: if you dislike a character, I challenge you to also find something good about them.
Mmmmkay. So. Who's your least favorite character (or couple), and why?
I'll get things started: I don't particularly care for Penelope.
First, the positive: IRL, I'm sure we'd get along well enough; she seems smart, bookish, and a bit snarky.
But then, there's the negative. The snark crosses too often and too easily into negativity. And there's the prudishness. And a touch of hypocrisy (she looks down on Wil's lack of career ambition,* but hello? Your ambition's to work in publishing, and your steps to get there are 1: quit bookstore, B: work at coffee shop, and 3: complain that you don't have your dream job?) And the fact that it's all wrapped up in a plain brown wrapper. She is, in short, the kind of person with whom you paint the town beige. I don't hate her, but as characters go, she leaves me as warm and fuzzy as, say, a ball point pen, and if you removed her from the story altogether a la Sara, it'd barely make a ripple. In terms of the regular cast (not peripheral/one-off characters like Lt. Potter), she's close to expendable.
*Not without some justification -- I mean, dude hadn't worked a day in his life.