[Big-Daddy]

So we all know fluoroantimonic acid has the lowest known pKa value. Let's call it the strongest acid, therefore. What then is the strongest alkali, by virtue of having the highest pKa (or highest pKb) of any known substance? Aqueous solutions of course ...

[Arkcon]

So you're looking for one of the superbases:  http://en.wikipedia.org/wiki/Superbase#Classes_of_superbases  Thing is, you mentioned the superacid fluoroantimonic acid, and that one isn't really an aqueous solution either.  So if you want the mineral bases, then the alkali metal hydroxides are more or less similar.

[Sophia7X]

So we all know fluoroantimonic acid has the highest known pKa value.

You mean lowest?

[Big-Daddy]

So we all know fluoroantimonic acid has the highest known pKa value.

You mean lowest?

Yeah, sorry - obvious mistake, edited 

[Big-Daddy]

So you're looking for one of the superbases:  http://en.wikipedia.org/wiki/Superbase#Classes_of_superbases  Thing is, you mentioned the superacid fluoroantimonic acid, and that one isn't really an aqueous solution either.  So if you want the mineral bases, then the alkali metal hydroxides are more or less similar.

OK, I didn't know that about fluoroantimonic acid, but let's say I am sticking to aqueous solution for now.

Any advance on pKa=36 (lithium diisopropylamide)?

[Borek]

This is moot, especially as you are limiting the question to water solution. Read on leveling effect.

[Big-Daddy]

This is moot, especially as you are limiting the question to water solution. Read on leveling effect.

I see. H₃O⁺ has pKa of -1.74, referring to the equilibrium H₃O⁺  H⁺ + H₂O I suppose, so any acid with pKa in water of less than -1.74 will act instead as having a pKa of -1.74. Same argument can be applied to any solvent.

The main question then becomes: what solvent levels off at the highest value of pKa, and at which pKa does this solvent level off?

By the way, am I right in thinking acids/bases have different Ka and Kb values in different solvents?