To make an objective list is impossible, keep that in mind when discussing.
Too true.
I would put Cyril Abati for consideration. Technically a master and he took the Trompe L'oeil too the next level with this
http://www.coolminiornot.com/59385?browseid=4748609 paintjob. Influencing many many people. This is another paintjob that has been very influential I think:
http://www.coolminiornot.com/52477?browseid=4748609 And he was early with NMM.
I can't believe I overlooked Cyril. Absolutely fantastic, and his style was definately influencial. I remember when i first saw that Space Marine you mentioned... that was back when everyone was still emulating the super-clean style (championed by the Eavy Metal team, and GD winners like Bobby Wong). It was so atmospheric and gritty, without being sloppy or rushed looking. I agree... I think that might have been a tipping point for many painters, and how we approached painting sci-fi in general.
I would not put Matt on the list for the simple reason that I don't think he has been that influential, same maybe with Dirk Stiller. I love the pieces you mention though and Matt is an all round top painter. But there are many others that have done very
inspiring things that have not been that
influential. Perhaps I'm marking words here but. Remy Tremblay for example mixed in techniques from movie sfx and have casted miniatures in transparent resin creating very realistic skin and has made some controversial minis like the baby face. Not sure how influential he has been though... But perhaps he will be.

I am for example thinking of doing something similar. Another very inspirational painter is Marjin Van Gils.
http://www.coolminiornot.com/forums/showthread.php?14579 this and this
http://i38.photobucket.com/albums/e101/merlinrossi/WorldExpo08004copia.jpg amongst others.
You might be right about Dirk Stiller's piece being "inspirational", but perhaps not as "influencial" as some other considerations. Darn... that piece still deserves to be mentioned somewhere for being so out-of-the-box and technically well executed.
Matt Parkes is also a major inspiration to many, but in terms of having influenced or changed the direction of the art form in a measurable way? Hard to say. He's certainly a superlative artist, who pushes our pre-conceptions of what the limits of miniature painting can be, but it would be difficult to say whether or not he's made us re-examine HOW we paint.
There are also plenty of painters who have been very innovative, but for some reason (lack of exposure?), their innovations haven't transformed miniature painting in general (yet).
And while I'm loathe to have yet another GW model on the list (there are just too many other amazing companies out there, but none of them have the combination of exposure, history, super-pervasive intellectual property, and sheer number of talented artists working on their models), I'm also thinking of including the Sanguinor paintjob from the Eavy Metal Masterclass book. I don't have my copy on me at the moment, and I can't recall which Eavy Metal artist painted the studio version of Sanguinor, but I can say that that paintjob has been copied and emulated by countless painters out there... beginners and journeymen painters for the most part, but still. It helps that it was a fairly well illustrated tutorial on painting NMM, published at the height of NMM popularity, in quite possibly the most widely sold miniature magazine on the planet, but even the GW haters have to admit that it was a pretty damn nice paintjob, and a decent step-by-step painting guide.
Bobby Wong's stuff was inspirational, and epitomized the super-clean, super vibrant painting fashion at the time. He just seemed to do what everyone else was doing, but just at a much higher level than everyone else. I don't know if that qualifies as a "Game Changer". I still love his stuff though, and he's not the only one who moved on from painting at the height of their achievements... years and years before him, Mark Dance was one of the only North American painters who travelled all the way to the UK to compete in the Golden Demons before it went international, and he dominated in many catagories. Then he just disappeared from the scene. My sources say that once he won the top awards in miniature painting, he went off in search of another hobby so that he could start from scratch. A quick google search came up with the following links, showing that he's now one of the dominant model railway enthusiasts in western Canada:
http://www.youtube.com/markdance63
http://www.flickr.com/photos/27907618@N02/sets/72157624106602402/
However, I think now that miniature painting is a much bigger scene than it was in the past, and there's so much more to learn within this art form, we won't have too many more "Mark Dance"s and "Bobby Wong"s abandoning the hobby for something else. At least, that's what I foresee. There's just too much to learn now, for anyone to think, "Oh, now I know EVERYTHING there is to know about painting tiny little men" and getting bored.