The "best" (subjective, of course) DRM I've seen so far was made by Iron Lore and sadly they are out of business today. It was for Titan Quest. It wasn't the "usual" DRM per se, but they did something to "punish" pirates, and to perhaps "convince" them to buy the game AFTER the pirating act was done, not BEFORE it (I.E trying to "prevent" piracy like the usual DRM methods do, and in vain for most of them).
What Iron Lore did was this:
1) The pirate downloads a copy illegally, done.
2) The illegal copy is installed successfully.
3) The pirate launches the game successfully.
4) A game is started successfully.
At that point the pirate thinks he's going to "own" the game for free, forever, wrong.
5) At some point in the game there is a DRM "check" that crashes the game back to the desktop if that check fails, and it would fail only if the copy is illegal (without outright saying that it was due to the executable or other files not being the original, of course).
So, basically, the pirated version would play normally until some point where it would always inevitably crash. The only "problem" with that method is that since it was not mentioned with a clear pop-up message of some sort that the crash was due to the game being cracked most pirates who were affected by that specific crash at that exact same place all went on the official forums to ask for technical support because they thought that the game was "buggy" and just always crashed there "for some reason" despite their PC being "stable", etc.
I bet that Iron Lore never saw that one coming though. They called it a night and must have thought that since it would always crash the affected players (pirates) would buy a legal copy because a "legal copy" would never crash. Nonsense. So that method was a nuisance for the image of the game (figuratively speaking) and the company. Still, in the end, the principle behind it was great, but it wasn't monitored properly, wasn't dealt with honestly by Iron Lore (not saying it crashed due to a failed DRM check in-game) and wasn't clearly stated which of course makes sense... in a sense. You don't exactly call bandits to warn them in advance that cops will be sent to arrest them, do you? Exactly.
But then again, in this case, if you don't tell the pirate that the game crashed at that specific point for the Nth time because the DRM check failed due to the copy being illegal then what do you exactly expect the pirate to think? And that's where things start to get fuzzy for the developers. What CD Projekt tries to do will not convince pirates that it would be too "risky" to download the game illegally. Even if they DO manage to find a few specific pirates here and there, what will those specific cases tell to the majority of pirates? There's tens of thousands of them out there if not way more than that all potentially going to download The Witcher 2 illegally.
In the best of worlds CD Projekt would "win" buy preventing ALL pirates from downloading the game illegally and instead buy it, then again, what exactly ensures that those pirates WILL indeed buy a legal copy exactly because they wouldn't be able to pirate it for some reason? Nothing is guaranteed. And that's what I myself will never understand... WHY do developers think that a successfully prevented illegal copy download automatically means that it IS a successful legal purchase... it's beyond me, because it's not the case. And that would be the actual goal of a successful DRM, right?
If you use DRM in your game then as a developer you want that to not ONLY to prevent an illegal copy download but ALSO want to ensure that all the copy that ARE being downloaded digitally ARE legally downloaded (Steam, D2D, etc), because otherwise what's the actual POINT of DRM as a whole to start with? If DRM ONLY prevents a pirate from downloading a game illegally and DOES NOT ensure that the same pirate will then BUY the game BECAUSE he/she wasn't able to crack it then DRM serves NOT purpose whatsoever, since the pirate will simply forget about it completely and call it a night and never buy the game legally since he/she wasn't able to crack it, hence no legal purchase from that same person, thus means that there was NO LOSS IN SALES...
But, heck, developers in general just don't get it.