http://www.moddb.com/mods/research-and-development

Let me explain my self.
Best is in ''' because I have not played every HL2&EP1&2 mod ever made but I've played alot but this single player puzzle mod is quite frankly the best I've played or seen.
(And yes I have played Minerva)
And It says HL2&EP1&2 because it requires you to install through steam Half-Life 2: Episode 2
to play.

Research and Development has the standard and class that I have not seen since I first played Half-Life 2.
Puzzle are unique. Yet complex enough to make you feel smart when you complete it.

http://www.moddb.com/mods/research-and-development

'Matt Bortolino's excellent HL2: Episode Two "action puzzler" has been released from nowhere!' - Vasity (Moddb user)

'Despite being a mod, it is better than the entirety of Half-Life: Episode One and most of Half-Life 2' - Destuctoid

'Even if you don't like the kind of games I normally suggest, I find it hard to believe you won't enjoy Research and Development. Get it here, or hit the jump for more theories on why this mod is so goddamned good.' - Destuctoid

'The mod's creator managed to take all the base mechanics found in Half-Life 2 and push them further and harder than I would have ever thought possible. Just when you think you've seen everything the game has to show you, it'll throw out something entirely different that will almost make you wonder if you're still playing the same mod.' - Destructoid

'I... feel guilty playing this game for free' - Destuctoid

'Seriously, I dunno why you're still reading this. Get Research and Development here.' - Destructoid