Ikea mattresses are so popular because of their really low prices, especially comparing with many other popular brands
However there are really many negative reviews about them because in their case low prices mean often low quality too, for example in case of coals and density. Some of their mattresses can be uncomfortable because of these reasons too, many models have limited comfort layers. It's often said that their mattresses can be toxic too but it's not true because their production and releasing are well-controlled.
Warranty for them is 25 years but in fact they last much shorter and start to sag fast too https://sleepmentor.net/ikea-mattress-reviews/
. Their mattresses not always good in being always cool and the most amount of models is good for leight and small people, not the heavy ones.
Despite of all these things mentioned above as the best ones in case of Ikea are considered such mattresses as Ikea Morgedal, Minnesund, Hesstun and Morgongava.
Hope that was helpful.