Here is my thinking:
(Between Senator Obama and Senator Clinton)
Unless Senator Obama implodes he beats Senator Clinton. He is way more charismatic and appeals to a broader base. He also has youth and is a way better public speaker. But primarily he hasn't had to take a stance on much. Senator Clinton has experience and toughness but she isn't liked by the anti-war left and is seen as an above average flip-flopper.

(Between Rudy Guliani and Senator Mccain)
To me this one is much much much closer. They are both about equally charismatic (though that charisma displays itself in different ways) They have also both flip-flopped a bit and have some questionables in their past personal lives and yet both are know for being fairly honest as politicians go. Mr. Guliani has a bit more notoriety with the average person but Senator Mccain has more political muscle. Both believe in things their base does not support but Senator Mccain seems to be just slightly more conservative overall on the issues. Age could be an issue (It hurt Bob Dole)

(Democrat v Republican)
I have said since before the last election that this presidential election would be the dems to lose. President Bush is wildly unpopular and any republican who could make it through the primary would be hurt by association. And yet it seems to me that the democrats may do just that.

Senator Clinton is the worst possible choice for Democrats. If she won the primaries and she would have to face off against either Senator Mccain or Mr. Guliani she would be in a world of hurt. The two republican candidates both do well with moderate democrats and independents. Senator Clinton not only wouldn't do as well in that department but her last name alone would mobilize republicans. I imagine quite a few of them would go out of their way to try to ensure she did not get elected.

Senator Obama is a different story. He is young, charismatic, and hasn't really been forced to do anything unpopular yet. Oh yeah, and did I mention that he is young (LOL)? His youth could be a major weapon against both the afforementioned republican candidates. Also unlike Senator Clinton he could feasibly win the majority of independents, although it is unlikely many republicans will vote for him based if nothing else on his anti-war stances as of late.

So...
Clinton v Guliani (Guliani wins big. People like him and he appeals to liberals,
he could be equated to fictional Pres. Josiah Bartlett)

Clinton v Mccain (Close one, though I give the edge to republicans
because republicans would mobilize and Senator Mccain
has a history of getting independents)

Obama v Guliani (Too close to call)

Obama v Mccain (Close but the edge goes to the republicans because
Senator Mccain's history, and political muscle, will be an
asset.) PLUS I LIKE HIM

On the upside. If these are the candidates we will at the very least be getting for once someone of unquestionable intelligence and good communication skills. No more Al Gore or President George Bush II type candidates.