This is tough, because I love Rayman. It's a fun, entertaining game, with great use of music and stellar animation for its time, and is also the #1 selling UK Playstation game. It's one of the most iconic platformers out there and its sequels are still being felt today in the Wii craze over Raving Rabbids or whatever they are. A brilliant and fun game, really.
Starcraft, however, is pretty much undeniably stronger. It's the best selling PC game of its year, it's sold nearly 10 million copies total, it's taken over South Korea in a way Kim Jong-Il can only dream about, and its one of the most popular online games in the world. It's a great game - it doesn't break the mold, but its one of the best games to use the mold. The distinct utilization of race was a big factor that, although it didn't invent, Starcraft used to great effect. It has one of the strongest video game stories around, and I can go on and on here, but there's no more need to. It's all these points and more that make Starcraft one of the best games of all time, and unfortunately, the fun, entertaining Rayman has to go out to the powerhouse that is Starcraft.