There are a few designers out there who only have a few games to their credit, but the quality and/or innovativeness of their games is huge. There are two that I want to point out who I think sometimes get overlooked when talking about the great wargame designers.
First is Courtney Allen. If you don't count expansions, Allen has only 5 games published under his name that I know of--but they are insanely good and ground-breaking games. First is Storm Over Arnhem, a game which shook up wargame design by getting rid of hexes and replacing them with areas of different sizes. It also broke up the turn sequence: rather than an IGO-UGO system, in Storm Over Arnhem players alternate moving some units in impulses. The resulting system is thus called the area-impulse system. This system hasn't taken off the way card driven games has, but it's had a major impact. One of the highest rated wargames out there, for example, is Breakout: Normandy, which uses this system. Also, MMP has a whole series of area-impulse games (some published, some in pre-publication still).
Even more innovative, though, is Allen's design Up Front. Up Front gets rid of hexes and counters entirely, shifting the game to cards for units, terrain, attacks, and other facets of infantry combat. To this day, Up Front is often worshipped as a great game--and I fully admit that I'm one of the worshippers. It's an insanely great game. Unfortunately, it's been out of print for a long time, and it's very expensive to acquire. I bought it in college, so I don't need to worry but I'd love to see it come it back. MMP has the rights to reprint it, but it's been lingering (literally a decade); I don't really understand why it's so slow. And oddly, despite the high praise, Up Front hasn't really spawned a lot of imitators. Some other games use parts of its design, but the only game that has really built on it in a big way is Allen's own Attack Sub, which ported the system to Cold War naval struggles.
The second designer I want to highlight is Craig Besinque. Besinque's games have all been block games, and although he didn't invent the genre he pushed in ambitious new directions, yielding some of the most highly praised wargames today. In particular, Besinque is best known for two brilliant games: Rommel in the Desert, and East Front (both from Columbia Games). Both are deep and rich games without being overly complicated; both have very clever and playable ways of simulating limitations on command and control. Besinque's latest design (this time for GMT Games) is Hellenes, a simulation of the Peloponnesian War, and it looks to be an outstanding refinement and expansion on the Hammer of the Scots-style block game.
So let's take a moment to appreciate brilliance of these two designers. I think they may have the highest batting averages of all wargame designers--only a few games under their names, but the quality of their games is staggering.
PS: Does anyone know what became of Courtney Allen? I've seen no evidence that he's designed a game in twenty years. I assume he moved on to fields that actually pay a living wage, but gaming is poorer for his absence.