How fast are the fastest TRNGs? Which ones have the most entropy?
This recent article claims 100 Gbit/s min-entropy, and gives reference to several >10 Gbit/s generators. These are lab experiments, where some quantum phenomenon is converted to digital with an Analog-to-Digital Converter. Much of the generator's entropy comes from the low-order bits of the ADC (it's acknowledged that an ADC with more bits increases the rate), thus I'm not so sure that most of the claimed entropy rate comes from the quantum phenomenon observed, rather than from (quantum) noise in the ADC.
It's dubious that such ultra-high speed is much useful in practical use cases, because a TRNG can be used to seed a Cryptographically Strong Pseudo-RNG, which can have high rate and (by definition) are indistinguishable from a TRNG. Even a 100 kbit/s rate (which would be easy with the practical constructs below) is fine to seed a CSPRNG with 256-bit state in under 3 ms, and very few applications need random numbers this soon after power-up.
Which ones are the most practical ones?
Common TRNGs in integrated circuits use ring oscillators, and/or noise generators compared to their mean, or combinations thereof. Their output is typically feeding a de-biasing circuit or/and a Pseudo-RNG.
The most practically important criteria for a TRNG in a crypto context is that it does not go, undetected, into a state such that it produces much less entropy than expected, by accident or because an adversary deliberately induced that (e.g. by spraying some liquefied gas on the gismo, connecting it to a power source with high ripple, putting it in latch-up with an ESD, or beaming some laser or powerful RF signal at it).
Typically, the hardest-to-get-right part is surveillance of the TRNG to detect that it fails, with two pitfalls to avoid: not detecting a failure, and false detection.
Is there a way we can rank random number generators ?
For cryptographic applications, the qualitative properties of TRNGs are ranked into functionality classes. The lowest, deprecated, basically is to pass statistical tests like NIST 800-22. Speed can also be a ranking criteria.