Can you recommend a reliable, safe crib mattress? (firsthand knowledge only please)

I am driving myself crazy looking for a crib mattress for my soon to be born child. Can you recommend something that won’t sag in the middle after only a few months of use? I would like a mattress that is really firm. Please do not respond with a bunch of internet research (I have already done plenty). I would like to know what worked for your baby. If you can link to the item that would be great. Thank you!