My favorite is Alder, but I also like Swamp Ash (which is just a term for labeling many different types of Ash). I do however think that in general the neck has a slightly bigger impact on the sound than the body has. But it depends largely on the construction. How thick the body and neck are (compared to each other also), how you join the neck and body, set neck, neck through and bolt on.
If you have a neck through, you basically have the neck material as the biggest and most important part of the body, you really only have "wings" on the side of the neck. You can also make the neck part that goes between the wings very large or just normal sized, same size as the neck. Some builders have a thicker part there so the wings will not affect the sound that much, I doubt anyone could tell the difference if they first played the guitar with Alder wings and then cut them off and glued Mahogany wings on instead, if the neck material is Maple it will always sound like Maple does. With other words it will remain the same.
The thickness of the body will affect the overall sound of the guitar more than the actual wood does. If you have normal thickness and Alder, let say 45 mm and if you divide that with roughly 2 so you have let say around 25 mm it will change the sound dramatically, it will sound much thinner and have less mids. The sound will have less "body" so to speak, not just physically ;D
It doesn't matter if you add Mahogany to the thin guitar it will never be as thick sounding as the physically thicker guitar anyway. The difference has to be big tho, it won't be any audible differences on just a couple of millimeters.
Maple and Alder is my favorite woods for guitars and basses.
My favorites are Basswood and alder. Although currently my main guitar is mahogany (Ibanez S 25th) , it doesn't make too much of a difference if you use a decent amount of gain and compression and delay in your rig. I think depending on the type of setup, pickups could be more significant than body wood.
There are actually 2 types of Ash used on guitars- (Southern) Swamp Ash which is the nicest and (Northern) Hard Ash. I wouldn't choose either for anything other than a Tele where I need that trebly twang. The lows are almost non-existent and the tone very limiting.
It depends on the genre/style/tone I'm going for but Alder is probably my allround favourite due to it's good but tight bass, very good low-mids, nice mid-mids and decent treble. It can be used for pretty much any style and a great player with excellent touch and finger can make an alder guitar sing.
There's something about a big slab of mahogany with a 1/2" maple cap for anything "rock" though.