Just to let you know, if you're counting JUST the development, it's Valve. Blizzard, while taking forever on Diablo 3, are NOT doing so for development. They're doing so because of testing to make sure they work out all the bugs.
Now, I know what you're going to say. "But testing is a part of the development process!". I know it is. The difference is that the major testing happens not only as each level/area is being worked on, but over all after everything is completed. The part that happens between level development is QA, or Quality Assurance. That's fine as it doesn't impede the time of development. However, the main beta testing (which, in Blizzard's case, I'd assume they do most of within the company due to the size and money the company is/has) doesn't happen till AFTER the game is completed. Any found bugs are then corrected and the game re-issued for more testing. This is a very lengthy process.
Now, you might wonder how come they can spew out a mass load of sequels/add-ons to WoW, while taking forever on D3. D3 has been built from scratch. New system, new style. You say that it's taken 10 years to make it? No. They did have a version that used D2's engine and style but scrapped it in order to start fresh.
However, the WoW add-ons use all the same engine and graphical style and capabilities. The only difference is creating the story for the add-on, adding in the new character/NPC images and placing any changes to the surrounding world, etc. It's like comparing a casual game (say, Angry Birds) to a massive, full length game (eg, a Final Fantasy-style game).
In all fairness, both are great companies that have produced some of the greatest games of all time. I'd rather wait forever for a perfect, practically bug-free, game like D3 will be when they finally finish it all and release it than get a game only to have the next game in the series released a few months later.