Obviously I can't tell you which mattress you "must" buy, as I try to be unbiased and neutral, allowing you to form your own opinion.
If you are budget-conscious and want a hybrid, then OTTY are definitely worth a look. Bear in mind the drawbacks of some memory foam/hybrid mattresses, e.g. they can't be flipped over, and their shelf-life is going to be <10 years in all likelihood.