We get a lot of posts from people looking for the "best" mattress, and unfortunately these questions are really difficult to answer. Mattresses vary widely by comfort and quality, and what is good for one person isn't necessarily good for someone else.

However! There are a lot of brands I'd recommend that you AVOID AT ALL COSTS. These are brands that make mattresses of unknown materials, have a consistent history of terrible 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 one of these is a much riskier proposition. So without further ado...

1. Nectar. I'd avoid them. We've gotten more complaints about Nectar (and the other brands associated with their parent company Resident Home) over the last three years than any other mattress company. I've seen material substitutions, poor quality, and reports of customer service ghosting customers after purchase. I don't trust them whatsoever.
2. Dreamcloud. Owned by the same company (Resident Home) as above. They use unknown materials, are constantly changing the construction of their mattresses, and have a terrible track record. Hard pass.
3. Puffy. Much like Dreamcloud, Puffy has a poor track record with customer service and material substitutions. There are no details on the foams they use, and I generally don't trust them.
4. Zinus/Lucid/Linenspa. These are generally 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 typically have fiberglass fire barriers. In fact, there are ongoing class-action lawsuits against Zinus for their use of fiberglass fire barriers. No idea how that will shake out, but I'd avoid them.
5. Molecule. I don't see this one mentioned a lot, but the last time I looked at their material specifications their base foam was only 1.5lb/pcf. As a general rule of thumb, do not buy a foam mattress with a base foam under 1.8lb/pcf unless it's intended for a guestroom or occasional use.
6. Comfy. We don't hear much about Comfy either, and I don't expect them to be around long. However, I'd like to make a special note of this brand as they work with the some of the most obnoxious affiliate marketers and astroturfers around. These are fundamentally dishonest business practices and I'd encourage consumers to take their money elsewhere. They also do not list any relevant details or material specifications on their products.
7. Serta/Simmons/Sealy/Stearns. I'd avoid these with the possible exception of some of their higher-end (and higher coil count) models. These generally use low quality foams, but can have some pretty excellent coil units. I would absolutely avoid the lower-end models though. These will quickly lose loft and support.
8. Ghostbed. Horrific customer service with a very weird return policy. Lots of complaints here.
9. Plushbeds. Plushbeds also works with some of the scummiest affiliate link spammers around. We've banned dozens and dozens of these folks over the last two years. There are too many solid latex options to support a company with dishonest business practices.

And those are the ones that come to mind right now. Any that I missed? What do you think?