What do you call a client who is one of the most important for your business? [closed]

We are a small company and we treat this as an advantage. We work with a very limited number of clients, so each and every one of them is super important for us. We can't afford to screw anything up with any of them because our business depends on them. It's by design.

I want to write about us and tell them that because of this decision they are becoming very important for us when we start working with them. I'm trying to figure out a good idiom for this or at least a short sentence that will work here. I've found

jewel in our crown

and

the apple of our eye

But both seem to be a little odd in terms of calling our clients that. We have rather laid-back communication so it's not about being a bit too familiar with them. It's not a problem to tell them we love working with them or care about them, but the idioms I've found seem a little old-school. Is there anything that will apply here better?

If your main intention is to express how crucial they are to your business, you could call them pivotal clients or key clients.

Or you you could use metaphors such as

You are the backbone of our business

or

You are the keystone of our business

However, it is not clear from your question which meaning exactly you are trying to convey. If you wish to emphasise the fact that they are precious to you, then you could say they are your lucky stars (since you mentioned you could be slightly familar with them) or simply your number one clients.