The worst of which are Valve's own Custom Executable Generation, and Denuvo, which is third-party but (last time I checked) Steam doesn't warn you about as being a 3rd-party DRM.
To be honest though, the Valve Custom Executable DRM is pretty easy to bypass. There are automated tools that make it pretty much trivial. I won't link them because it's technically illegal in the US.
Denuvo is the most difficult one, yes. It's technically not DRM, but anti-tampering software that wraps around another DRM, that said it is really the worst.
Yeah, Steam's DRM implementation is ancient and well known to be compromised. They never really push it, but publishers demand it to "feel" safe. The biggest hook is that many people prefer games on Steam due to the added benefits.
It's very, very easy to remove the DRM yourself, though. It's more a legal formality than an attempt at DRM and is pretty much never updated.