I see a lot of posts from people looking for the “best mattress” and, unfortunately, these questions are pretty difficult to answer. Mattresses vary widely in terms of comfort and quality, and what’s good for one person isn't necessarily good for someone else.
However! There are a lot of brands I would recommend that you avoid.
These are brands that make mattresses of unknown or low-quality materials, have a history of poor customer service, or are suspicious in other ways. This doesn't mean that their mattresses are always bad (or that you can't learn to sleep well on them); it just means that purchasing from these brands is generally a riskier proposition.
(I should also point out that the intent of this list isn’t to punish these companies, but instead to highlight problematic practices and work to drive the industry towards improvement. If you work for one of these companies - here are the things to improve on. Also if you think I've made a mistake here - please let me know!)
Nectar/DreamCloud. I'd avoid them. I’ve seen more complaints about Nectar and DreamCloud (and their parent company Resident Home) over the last years than any other mattress company. I've seen material substitutions, poor quality and fiberglass, and reports of customer service ghosting customers after purchase. They’ve also been in trouble with the FTC and appear to be involved with spammers all over Reddit.
Zinus/Lucid/Linenspa. These are typically the cheapest mattresses on the market and imported from overseas. These are made of unknown materials (and likely bottom of the barrel in terms of quality), and often have fiberglass fire barriers. Most reports of fiberglass contamination on Reddit are from these brands.
Ghostbed. Ghostbed has some interesting designs. There aren’t many companies on the market that are using a full sheet of gel in their constructions. That said, they’ve historically been not great with customer service and have some very weird language in their return policy. There have been a lot of posts about this.
Puffy. Haven’t heard much about Puffy lately, but they’ve historically been associated with poor quality, poor customer service, and Reddit spam. Still, they’ve recently updated their models, and it’s possible that they’ve improved their business practices as well. I'd put them at a very tentative maybe?
Molecule. I've been trying to be fair to Molecule after seeing some potential improvements in their recent lineup, but I've still not seen enough info to really take them off this list. They previously used a 1.5lb poly foam as a base layer in their foam models, which is a fair bit lower than what I usually recommend for support layers. Still, the details on this appear to have been removed from their website, so it's possible that they've fixed this? Not sure. I should also say that I think their hybrids generally look okay. I'd also put them at a maybe.
Sleep Number. I actually don’t hate airbeds (more here). These support differently than pocketed coils, but they can be a good option for some people. They also have a lot of potential for people with chronic pain or people that want an adjustable sleep surface. I think there's room for an airbed company to succeed in the market. I'm not sure if that's Sleep Number. Their higher-end models are probably okay, but their lower-end ones are pretty under-spec'd and over-priced. Still, I’d like to see them turn it around.
Cheap Major Brands. I would avoid the “value” options from Serta, Sealy, and Beautyrest. These tend to use low quality foams and low coil-count units and are likely to quickly lose loft and support. If you do purchase from one of these companies, I'd advise you to start in their mid-range and go up from there.
Brands associated w/ Online Spam. Finally, I’m going to make a broader point about being careful with brands associated with spam and covert advertising on Reddit. These are brands that use fake accounts and bots to artificially increase mentions and manipulate SEO to sell junk to consumers. I think this is wrong. I actually think these practices harm people. There’s a right way to do online advertising, and that’s to be forthright and honest about it. You should be suspicious of random accounts popping up out of nowhere to leave glowing reviews on products in threads all over Reddit. These are everywhere.
And finally, talking about these things invariably brings up the question: well what should I buy?
This is a bit difficult, but I can simplify it somewhat. You should look for brands that are upfront and honest with their material specifications (including foam densities, coil counts, and coil gauges) AND have a history of solid customer service. These are often smaller brands or local stores. That’s it. And again, you should generally be suspicious of online reviews and highly upvoted threads, which can be easily manipulated by dishonest people and/or outright bots.