What is the best mattress for a big guy to get?

I am 6'5", 240 lbs, my girl is only 115, but the bed has sunken in the middle. I have rotated everything, but still no good. I need to buy a new one but dont know what to get.

Answers:
When I went to a store that makes custom mattresses, I asked about sagging and big bodies and what type mattress is best. The man there told me that the best is a latex (foam) type mattress. He said all the spring type mattresses will sag, in time.

My husband is large and now has a latex mattress. He likes it a lot. It is totally flippable, turnable, and recovers it's shape readily. You can order such mattresses to whatever softness level you prefer, and however thick you want.
I can't recall the name of the company I ordered from, but these links look pretty informative:

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