AH-64s don't do CAS, technically they are Close Combat Attack (CCA). They do not require JTAC control. The AC-130 cannot survive in a high threat environment, and is typically prohibited from flying at all during the day. The Reaper can do quite a bit of what the A-10 does, but doesn't quite replace it. It lacks the gun, but partly makes up for it by being able to hang around for a ridiculous amount of time.
The F-22 will probably not ever be used for CAS, but the F-35 can do most of the mission set (as we have just seen in our recent exercises here as I mentioned). The low and slow thing used to be a big factor (fast movers couldn't spot stuff due to being too high and fast), but now that all fighter aircraft carry Sniper pods and the like, that is not an issue anymore.
The F-35 can do from 20k most of what the A-10 can do down low (the big exception is the gun of course). The gun is something that nothing else but the A-10 can do in quite the same way, no question about it. Even that though may not be as big a deal as you might think. With the racks filled with small diameter bombs (SDB-II), a single F-35 could kill a dozen tanks or more, with little risk to itself, and almost no chance of missing. The A-10 doesn't carry enough ammunition to do much more than that (they have enough for about 17 seconds trigger time), although you could make the argument that if the A-10 was carrying as many SDB's, it would still have the gun on top of that, and that is a good point.
In the end, there is no question, there has never been, and will probably never be an aircraft as good at CAS as the A-10, but it may itself be forced to perform CAS from a high altitude due to all the modern threat systems out there. If it is forced to stay high, then the F-35 is going to be better. Even at high altitude the A-10 may not be survivable with the latest SAM's that are starting to proliferate. Note that being the best spyplane ever did not stop the SR-71 from being retired.
Here is to hoping that lack of money (a small amount in the federal budget really) does not prematurely end the A-10's reign as guardian angel for our troops. My old commander (who was an A-10 pilot) had an A-10 decal on the windshield of his HMMWV with the caption "for when you are all out of hooah!"
