I will start out by saying that I have posted on this subject once before. But that post is probably lonmg gone by now, and since this new, nifty section has been created, I shall post again with the hopes that Slayers fans may find this.
Let us begin...
Xellos is mazouku. This is a well-known fact. And at this point some of you may be starting to grumble about how you know where I'm going with this and it's not true, but... oh well. It is true. Mazouku are beings of negative energy, "like a high-density ghost" as Lina put it. They have no actual physical bodies, thus they can't be hurt by a regular sword or anything. Their bodies exist on the astral plane and the astral plane alone. This is also a well-known fact (or at least I hope so).
Mazouku feed off negative emotions: anger, terror, pain, etc. Good emotions make them uncomfortable (and can even kill them if directed right). So they cannot love. Some of you say that they can because dragons can hate things, but mazouku are unique creatures. They are astral beings, the dragons are not. And the dragons are servants of the gods, not gods themselves. Mazouku are, essentially, demons. They must obtain negative emotions or die.
So, brief tutorial is over, let's begin round two!

Lina- I could kind of see this happening, although I don't officially support it. Lina causes enough death and destruction around her for Xellos to be able to feed off of. However, I highly doubt that Lina would ever abandon Gourry for a fling (for that's all it could be) with Xellos, considering that she is reluctant to get caught up in the affairs of mazouku (oh look, I kind of made a pun... whee ). Besides, Xellos doesn't love creatures. He tortures them and eats their emotions. Yes, he kind of toys with Lina (the two kind-of kisses) but that, as one intelligent person who's name I cannot remember put it, was to distract her from asking questions. Both times she was asking hard-to-answer-without-revealing-mazoukuness questions, and both times indirect kissing stopped her dead in her tracks. I would do the same.

Amelia- I don't know who thought this one up, but this doesn't stand a snowball's chance in hell. Amelia fights for justice. Well-known fact nunber 795.2. When she discovered that Xellos was a mazouku, she nearly passed out... or freaked out. Whichever. She tried to cure him for crying out loud. While she can accept that he travels with them, (to take that instance of that town being attacked in Try) she was not happy about the fact that he knew said town was being attacked and did absolutaely nothing to help. Besides, hanging around Amelia too long would make anyone naseaous.

Zelgadis- Yep. There's a controversial one. And the sad thing is, that although I would never want to see it, it could happen. Think of the angst, helplessness, and sheer outrage Zelgadis would feel if Xellos... ah... cough. Yeah. It'd be one hell of a feast. And since Xellos really isn't a he so much as an it (no physical body means having no definitive gender) it can't really be called yaoi, could it? Besides, this would be an incredibly short term relationship... probably ending in somebody's early death.

Filia- On a scale of popularity, this one is right up there with Lina. There are a lot of people who support this one simply because "they're so cute together." Look, Xellos would have killed Filia if he had to at the end of Try (although the entire thing was a plot merely to get Filia to try and blast him with Holy magic). He's killed thousands of her kind. Yes, she does seem to have a knack at getting to him (namagomi) but, again, that is no basis for a relationship. I've seen only a few fanfictions that could pull this off, but all of them involved Xellos no longer being a pure mazouku. And while there is a fine line between love and hate, I doubt that Filia will be crossing that one anytime soon.

And now, the most likely pairing of Xellos goes to...
Juu-ou-sama!!! xd
Why you might ask? It's simple. They are both mazouku. Granted, it would have nothing to do with love (the way humans understood it anyway) but it could be formed on the basis of lust. And before anyone out there starts saying that it's incest or whatnot, Xellos is a piece of Zellas, not her child. There is a bond between the two, considering that Zellas is Xellos' creator and as such she commands his utmost loyalty. It really isn't too surprising that they could have sexual encounters if you really think about it. It could even strangthen their relationship!

And I've ranted enough for now and since it's around 6 am my thought processes are getting a little hard to follow. So I'll leave you there for now. I do hope that I'll come back to some intelligent responses and maybe even a good debate or two (nothing like debate in the morning... or afternoon depending on when one wakes up, to get the brain working. It's like the alternative to coffee). So, please reply or even better, disagree. It's always more entertaining that way!