I voted Maggie Walsh and Riley.
I think they could've used Maggie Walsh to serve Buffy's view on being the slayer. Yes Buffy has to slay vampires, but that doesn't mean she has to treat them like animals. I think vampires and slayers have the understanding that one of them is going to kill the other (and it all depends on the fight) and they accept and respect that fact. Therefore Maggie Walsh would serve as an ethical villain rather than a mwahaha one. Robot Adam could even be used to give Walsh some emotional depth eg Adam was Walsh's son who was killed by a vampire (hence her view of them as nothing but animals), and in her grief she created a replica of him and her attachment to the replica is both crepy and tragic.
Riley could be Walsh's secret toy boy lover (perhaps with a subtle Oedipus complex) and she charges him with the mission to get close to Buffy and keep tabs on her. Riley begins a relationship with Buffy, and this could be played as either him just using her or he genuinely does begin to fall in love with her. Either way, it might make Riley a little more interesting.
My thoughts on the other suggestions:
Ethan Rayne- Wouldn't work. His story purpose revolves around Giles rather than Buffy. Add in the fact that he worships "chaos", he hasn't got the calculating nature to be a big bad. He works better popping in and out of the show to annoy Giles.
Sunday- Wouldn't work. She wouldn't be able to carry a whole season as a big bad. However, as a mini-bad or small arc villain, she'd do well.
The government- I disliked the government being involved with the initiative as it made the whole thing "too big" if that makes sense. It would've worked better if the initiative was something that was based off of old discarded military intelligence eg the "Destiny" flashabacks on the submarine, but that the government weren't actually involved in. It would also explain why Riley was in such a high ranking position (slept his way to the top) despite not being old enough to have the years of experience that he would need in the actual military. Basically, the initiative is Maggie Walsh's thing not a government thing.
The gentlemen- Would not work at all. There is no way they could carry a whole season as a big bad. For a 2 or 3 episode arc, yes, but as a whole season, no.
Kathy Newman- See above.
Faith- Wouldn't work. She's not got the brains to be a big bad. She's more of a "dragon" type character. Any villainy Faith could've had, they kind of dealt with in Season 3, to carry it on in Season 4 would feel like a rehash.
The First slayer- Wouldn't work. She's not got the presence to loom over the season as a big bad. Also, the slayer has been built up as a force for good, to have the first slyer be evil kind of goes against that. However it would be interesting for Buffy to find out her power comes from evil.
Spike- Without Dru beside him to provide the emotional aspect, Spike is kind of adrift in the villain stakes. He has no motivation to cause trouble for Buffy on a larger scale (the trouble he caused Buffy in Season 2 was related to Dru in some way). Spike would work as a frenemy who helps and hinders Buffy depending on which side pays him more, but he has no reason to be a big bad.
Jonathan- He's too emotional and conscience filled. He needs a Warren to lead him down the dark path of villainy.