The quantum annealer is least powerful and most restrictive form of quantum computers, it is the easiest to build, yet can only perform one specific function. The consensus of the scientific community is that a quantum annealer has no known advantages over conventional computing.
Application: Optimization Problems
Computational Power: Same as traditional computers
Difficulty Level: A very specialized form of quantum computing with unproven advantages over other specialized forms of conventional computing.
The analog quantum computer will be able to simulate complex quantum interactions that are intractable for any known conventional machine, or combinations of these machines. It is conjectured that the analog quantum computer will contain somewhere between 50 to 100 qubits.
Applications: Quantum Chemistry, Material Science, Optimization Problems, Sampling and Quantum Dynamics
Computational Power: High
Difficulty Level: The most likely form of quantum computing that will first show true quantum speedup over conventional computing. This could happen within the next five years.
The universal quantum computer is the most powerful, the most general and the hardest to build, posing a number of difficult technical challenges. Current estimates indicate that this machine will comprise more than 100,000 physical qubits.
Applications: Secure computing, Machine Learning, Cryptography, Quantum Chemistry, Material Science, Optimization Problems, Sampling, Quantum Dynamics and Searching
Generality: Complete with known speedup
Computational Power: Very High
Difficulty Level: The true grand challenge in quantum computing. It offers the potential to be exponentially faster than traditional computers for a number of important applications for science and business.
Source from IBM Research