Whatever you get, this is a thing to spend money on that often gets overlooked.
If people don't get a good sleep at your place, you're done. If they have a great sleep, your reviews bump up. Absolutly do not get a cheap <$100 bed that creaks and throw on the cheapest mattress you can find.
Just keep in mind that mattresses are 90% marketing and 10% product.
I have had great success with the Wayfair 'Mattress in a Box' - the Wayfair Sleep brand has been great. Just check the reviews. You can get these for $150-300, and they seem to always be on sale. I have two beds in our airbnb, so I went with one more firm and one medium firm. There is no 'one size fits all' mattress, because people's preferences vary greatly.
I do like the hybrid foam mattresses that use the poly coils (instead of metal coils). They sleep cooler. Another benefit to these types of mattresses is they do not require a box spring (just another item to break, clean, conceal with a skirt).
Be sure to get a waterproof, bug proof mattress protector as well. No need to pay for name brands like Sealy, Tempur-pedic... all you'll be doing is paying for their TV commercials.