Cooling performance generally goes chunk of metal < heatpipe cooling ~ small sealed water coolers < "real" water cooling. If you need the lowest 24/7 temps, a real water cooling loop is the best way to go.
The BEST 24/7 cooling would involve a TEC or "Peltier" which would then be cooled by a water cooling loop. LN2 or dry ice can get a chip colder, but not for 24/7 use. Again, if you give us the point of the question I'm sure we can give you a better answer.