I remember I was first wow'd when I saw those dark vehicle army levels in Super Mario Brothers 3. I still love that idea.
Then I was completely blew away by the Super Metroid intro. I even watched the start up screen and gameplay shorts for awhile I was so dazzled by it all.
I went crazy with Warcraft 2 when I found out it had a map editor. I made up 250 maps. Minecraft sucks when compared to map editing.
Super Mario 64 psyched me up when I made it to the first boss who you had to throw down a big hill. That was the shit.
StarCraft. Just StarCraft. I fell in love with the bloody demo, replaying those same 4 levels about 20 times. Still believe it has the best style and music of all RTS's.
Lineage 2, my forever favourite MMO (well the classic chronicles), sucked me in when I saw it's great graphics (at the time) and awesome musical scores. I especially wow'd when I saw this giant beast called the Orphen wander around the low level area's, and people think WoW did it first.
Uh, fuck it. Golden Eye, Ocarina Of Time, Perfect Dark, Super Smash Brothers (the ad), Body Harvest, Mischief Makers, SSB Melee, Metroid Prime, beginning of Skyrim, and some others I probably forgot.
Then there's Half Life 2 which I first played in late 2012 (since I'm not a fan of shooters). Didn't think it would have so many awesome segments separating action and puzzles. Especially loved the boat to the prison break in, and the final part.
And the one that possibly surprised me the most, the original Half Life, which I only just played early this year. I did not expect that crazy intense adventure, especially the last 'area'. Didn't think I would be putting it above HL2 either.