I was born 2 weeks after E3 1995, so I'm not exactly too sure about who won it.
Sony won it. It's fair to say that no company has ever "won" E3 at a level that Sony won 1995 in any year since and no company will ever approach it. They went from nothing to destroying Sega's console future and positioning themselves to lead the industry for the next 25 years on the back of that "$299" announcement.
Other companies have had better lineups, better hardware, cooler presentations. But "$299" will forever be the quintessential E3 moment where everything changed. It was basically the birthdate of modern gaming.
Yeeeeeeppp. 95 was a scary scary time for video games. Sony was a giant tech company trying to force its way into the video game business, in the same way that Google is trying now. Nobody at the time knew how Sony entering gaming would affect things. Would they improve the industry or hurt it? Sony just barreled in train wrecking Sega, and pushing Nintendo to a distant 2nd place. Meanwhile, companies that had been fairly loyal to Nintendo suddenly dropped them like a bad habit, and published almost exclusively to Playstation. Imagine if Sega, Activision, and Bethesda just suddenly started publishing all their games exclusively to Stadia, and nowhere else.
At the time, I blamed Sony for Sega's console decline. In hindsight though, Sega had been digging its own grave for years. Sega CD, and 32X were both poorly supported, and split the console base, between to expansion kits. Saturn was a frankenstein of a console, lazily slapped together from parts Sega had lying around.