The original nelspot based markers were the start. Almost all original pump markers were derivatives of this design.
The quality of construction and success of the Line SI Bushmaster made it a favorite for tournament teams and pushed other manufacturers to follow suit. For that reason, I think it deserves acknowledgement in the category of game changers. I know the phantom also came at around the same time and had a lot of support, but the Bushmaster hit the market slightly ahead of it and garnished most of the fame due to its use by the top tournament teams. It was that fame that pushed other manufactures and changed the game.
Glenn Palmers Hurricane deserves mention. It was the first really successful pneumatic semiautomatic paintball gun. The autococker almost deserves mention. It is the semi-auto marker that most people know from the early semi-auto years. But, the autococker is only a modified version of the original Bud Orr Sniper with added pneumatics. Guess what? The pneumatic design was a copy of those done by Glenn Palmer. For that reason, I think that Palmers guns deserve mention as ones that changed the game.
The Automag is another early semi-auto marker that changed the game. It was so much faster than any other semi on the market when it came out, that it took over as the marker of choice at most of the tournament series. It was the gun that started the rate of fire arms race. If it had been introduced with a level 10 bolt system at that time, I doubt anyone today would even know what an autococker was.
The Spyder was a game changer because it introduced a reliable low cost semi-auto marker to the masses. The Spyder was based on the common stack tubed blow-back design. It wasn't a high performance gun, but it satisfied a market for casual users that allowed them to play with semi-auto markers at a price often less than that of a pump marker.
The Shocker was a game changer as it introduced electronic control to semi-auto markers. This started a whole new arms race because it allowed operators to fire guns faster than they could pull the trigger without actually firing fully automatic, which was illegal in most tournaments.
The Angel deserved mention because it introduced quality construction for electronic semi-auto markers. It was the first really fast electronic marker and set the bar that others tried to match, although the high price kept it as a high end gun that only serious players could afford. Even though people used other markers, they really wanted the Angel.
The Ion deserves mention because it, like the Spyder, introduced a low cost electronic marker to the masses and forced other manufacturers to lower their price point to compete.
There are other nice markers out there that have been around for a long time and are well known. It could be argued that they are game changers, but most are copies of other designs and didn't really influence or change the sport of paintball.
Except for the Automag in front, its usually the man behind the equipment that counts.