I’m surprised how often this question comes up. But I’m asked this relatively commonly.

First, a very, very brief précis of the main options

1. Reconstructed Koine (Buth or Buth-similar options)
2. Erasmian (US)
3. Restored Attic (Allen-Daitz)
4. Modern
5. UK traditional

Erasmian is what has been taught ‘traditionally’, i.e. for a few hundred years, and is dominant in US circles. It is often typically ‘infected’ with American-English vowels, which makes it even further removed from both Erasmian and historical accuracy. Both (1) and (3) are serious attempts to reconstruct and produce the sounds of Greek as pronounced in the 1st CE and 5th BCE, more or less. (4) is a recognition that Greek continued to evolve, and is in wide usage among non Anglophone contexts. It’s also quite appropriate for Byzantine texts. (5) I only mention because it exists as it’s own relatively idiosyncratic, but prevalent for a long time, system.

What should you use? Here are my principles:

1. A pronunciation system that is historically accurate for the period that is your major interest.
2. A pronunciation system that enables conversation with other speakers.

The first of these makes good sense – if you’re reading Imperial/Koine texts, you probably ought to read with a scheme that matches. If you mainly read classical texts, by all means use a classical pronunciation. If you predominantly read Byzantine texts, I’d shift one’s pronunciation to Byzantine or Modern.

But, I wouldn’t suggest trying to alter one’s pronunciation based on the period of text. So, I read classical texts with a Koine pronunciation. Which is, by the way, what I imagine the 4th century church fathers did too! I can manage a classical accent, though never well, but I don’t normally try to.

The second principle is an acknowledge that Greek should be spoken and ‘lived’, and if you’re in a context where a different scheme prevails, you should consider accommodating. E.g., I teach a class where the students are used to a pronunciation much closer to Modern. I don’t fully accommodate, but I do partially accommodate, and I don’t error-correct on pronunciation (or on other things, really).

Getting a good base in one pronunciation, you can understand others, if you’re mindful. Just as I can understand a Latin speaker with ecclesiastical (even if, to be honest, it does grate me a little), I can converse (not at a high level!) with someone using Erasmian or Modern, provided I’m mindful of the difference and mentally adjust a few words.

In sum, I think good reasons for using practically anything except Erasmian (and least of all UK traditional!), but anything else just stop worrying about it so much, you have much bigger problems ahead of you in the language learning journey.