Asics are currently the worlds most powerful mining hardware. ASIC miner is an acronym which stands for Application-specific integrated circuit. These devices are built for extremely specific use cases. In the case of cryptocurrency and altcoin mining, ASIC miners are designed and built to mine a specific cryptographic algorithm such as SHA256 or Scrypt. If you are a miner using this hardware, you can solo mine or point your hash to a mining pool which gives you access to a pool of hundreds or even thousands of other miners. Many of the more popular underlying blockchain algorythems are so popular that many different coins are built to use them. For example, the scrypt algo is used by Litecoin, Mincoin, and Dogecoin. If you have an ASIC Scrypt mining device, you can mine any of these 3 aforementioned coins. If you have a SHA256 based ASIC, you can mine Bitcoin, Bitcoin Cash, or Namecoin.
The largest manufactures of ASIC mining equipment are currently chinese based mining companies. The most popular brands in the worlds include Innosilicon and Bitmain. ASIC mining equipment can be difficult to acquire since so many coin investors around the world want to built bags of coins and get exposure to the global cryptocurrency markets. It is very common to pre-pay for equipment and then be forced to wait many months before the equipment is delivered. In addition to these long wait times, hardware can be faulty upon arrival or not allowed to be exported to certain countries or jurisdictions. In closing, the worlds most powerful mining hardware is ASIC miners.