I was initially very worried about Absolution, but I'm slowly warming up to it due to the facts that a) almost every recent mention of the game by the GB staff has emphasized that the game still has what made Blood Money great and b) the GB guys have shown in discussions of the series that what they consider BM's key appeal matches what I liked about it. That key appeal specifically being the exploration of the environment in order to concoct Mouse Trap-like murder plans.
I know there was one game in particular that showed up in a ton of different bundles during some of the earlier Steam sales, but I can't remember which one. If those count as multiple copies, though, then whatever game that was (it may have been Portal...?)
Strictly speaking in terms of hours clocked, I'd say the NY Times Crossword Puzzle game for the DS. But only because for about a year, it was my go-to game to play in bed to help me sleep, so there have been many instances where I'd wake up the next morning to find my DS open in my hands, still on. This also applies to Platinum Sudoku.
One thing I've found to be surprisingly effective (in places like early Act 3), is Locust Swarm with Pestilence. I'm only in Act 1 Nightmare myself, so I can't really say whether its effectiveness holds up later on, though.
I always thought that I was just missing something when nothing I tried for mouse 1 and 2 skills seemed as effective (for most situations) as splinter darts and dire bats. Maybe that's simply how it is.
Alright, that's what I figured. The only other reason I could see for holding off a little while would be waiting for any game breaking bugs to get patched (like with Skyrim), but I haven't heard too much about D3 on that front. Aside from its embarrassing launch day server woes, at least.