If you're intention is to get the chips hashing as fast as possible, ignoring power costs, then a TEC (Thermo electric cooler) plus a massive heatsink (even better a water cooled head on the back of the TEC) is the way to go. You can get the temperature of the chip well below ambient, which means you can get it colder, while still moving heat away just as air or water. Next option is an oil bath, which unless you can get the oil temperature well below ambient, then it can only move the heat but not cool the chip to super lower levels because you can only cool it to ambient; unless of course you refrigerate the oil, or have some cascade of heat pumps.
 You can buy water-cooled TEC plates there!
 Oil cooled pc in a fish tank

If along with electricity you have free time on your hands you could build a ASIC dry ice pot. This is a large thick copper cylinder with one closed and one open end, the closed end sits on top of the chips and into the cylinder you very carefully place acetone and dry ice chunks and make a "slurry" and its super super cold! But you have to feed it dry ice for 24/7 so it doesn't overheat from evaporating the dry ice off. Some people even use liquid nitrogen in the pot.
Good picture of a dry ice pot

If you have a lot of free electricity, time, or money you could buy an entire AC unit for you asics, or build your own. This uses a "head" akin to a water cooling "head" that is placed on the chip, but instead of water, freon or some suitable chemical is ran through it. It is compressed and held at some pressure at a liquid, it is then pumped onto the hot chip, and evaporates taking heat with it, then it is compressed with a compressor and cycled back through. These can get below ambient too.
A phase change system for a computer with one head

The coolest thing you could do if you have a lot of all of that stuff we all want a lot of. Then you could build or buy a thermo-acoustic cooler. This thing is bad-ass, it can get within a few degrees of 0 kelvin, but I don't think it moves much heat at that point. But you could find one suitable for the chips heat output, and not have to feed it dry ice, or freon, or air, or water. It uses sound pressure waves to cool a "stack" of straw like cylinders, it works super well and its super cool. Ben and Jerry's actually started using them in all of their freezers for ice cream and to make it because they are so easy, cheap, environmentally friends, and super cool.
Thermo-acoustic cooler article/link

But way before any of those become useful, i think you'd reach the chips physical limits. I have no idea about your specific hardware, but first run the chips and measure their temperature at increasing clock speeds and voltages, see if cooling is necessary.