I'll draft: Sakimoto, Yamane, Mitsuda, Yamaoka
If anyone actually cares for elaboration, please say so, otherwise I'll leave it at that. But those are four I've listened to extensively and don't really see/hear the big deal. Although it must be said I have a lot more respect for the last two than I do the first two.
Tempted to say Shimomura, Hamauzu and Nakano, as all have just grown off me completely over the years.
I'll second Iwadare as well...Much of his Grandia 1 and 2 stuff was excellent, but as far as a career goes...I have doubts with everything else of his I hear. And Sakuraba got by on the strength of a couple scores for way too long before I stopped seeing what the big deal was.
*waits for someone to say Uematsu*
Edit: Okay, since two people mentioned JDK, I need to muster some sort of defense. I think for what they set out to do, they accomplish it very well. Their music (original or arranged) always seems to embrace more of an old-school feel and isn't shy that it's out to provide more of an anthemic backdrop to the action, that is ripe for being arranged. I think there's a bit of honesty in there in trying to make deliberately fun compositions, whereas a lot of groups try to make their music sound more important than it really is. I find it pretty refreshing, especially compared to the empty orchestral pomp and atmospheric crap of so many games today. I find soundtracks like Sorcerian and Ys tend to be pretty accurate and quality examples of what people think of about VGM in the late 80's/early 90's (and yes, I mean that in a good way)...They're right up there with Rockman for me. And arranged better to boot.
The problem with saying "JDK" is you're talking about an entire sound team that has various members (including Yonemitsu, who may or may not have been a member) and different eras. Now the CURRENT JDK, I'm becoming far less fond of and I don't really think they have the charm that the older one did (which was deeply rooted in 80's synthpop/hard rock).