Both KOH and NaOH are pretty soluble in water:

A saturated solution of KOH in water at 25C and 1atm is about 22M (121g in 100ml)

A saturated solution of NaOH in water at 20C and 1atm is about 28M (111g in 100ml

Other compounds are infinitely soluble in water, but this is offset by higher molecular masses, such as sulphuric acid. If you 'dissolved' a large amount of which in one molecule of water the solution would be 19M, and the lower molecular mass nitric acid similarly 'dissolved' would sit at 24M

At the extreme end of the scale - a large volume of water (low molecular weight, high density) 'dissolved' in one molecule of ethanol would have a molarity very close to that of 'pure' water, fairly high at 55.5M (at room temperature, pressure, etc). 

Both KOH and NaOH are pretty soluble in water:

A saturated solution of KOH in water at 25C and 1atm is about 22M (121g in 100ml)

A saturated solution of NaOH in water at 20C and 1atm is about 28M (111g in 100ml

Other compounds are infinitely soluble in water, but this is offset by higher molecular masses, such as sulphuric acid. If you 'dissolved' a large amount of which in one molecule of water the solution would be 19M, and the lower molecular mass nitric acid similarly 'dissolved' would sit at 24M

At the extreme end of the scale - a large volume of water (low molecular weight, high density) 'dissolved' in one molecule of ethanol would have a molarity very close to that of 'pure' water, fairly high at 55.5M (at room temperature, pressure, etc). 

As I recall, CaCl2 goes up to ~50%. I don't know about the molarity, though. Sugar can go higher than 60% (Medicinal syrups are ~64%).

Water itself has the maximum molarity in a liquid (55.5 M).

Follows HF, with 50 M. If you don't count water itself, as HF is freely soluble in water, any solution from 0 to 50M can be achieved and this could be the highest concentration solution.

Sucrose has a solubility in water of 211.5 g / 100 ml (20 ° C). Water and sugar are miscible in all proportions, so, if we also consider solid solutions, the solubility of sugar in water can be extremely high.

Water has a very low molecular weight, so its molarity in itself is one of the highest.  If you want to go to extremes, the highest concentration I can think of is the concentration of water in any miscible liquid.  Water by itself is 55M, so if you were to place a drop of alcohol or acetic acid into it, the concentration of water would remain essentially at that value: 55M. 

Since this question pushes the boundaries of what we mean by dissolved my vote is for liquid mercury at a density of 13.59 g/mL and and MW of 200.59 g/mol that yields 67.75 M. It's a liquid and thus is "dissolved". Of course that leads naturally to molten salts, molten metals and the like - so it's possible that mercury is not the final say and the most concentrated "solutions" are alloys.

The liquid lithium (500 g/L) has maximal concentration (71 M), I suppose

mmm. if molten metals far from RT are valid, liquid beryllium have a density of 1690 g/cm3 at 1300C, yielding a molaruty of almost 188 mol/L.

In the class of solutions of a salt in water, lithium perchlorate is pretty extreme too with a boiling saturated solution being 28.3 M

(bp of saturated solution 120 deg C, solubility 3000 g/L, MW=106)