This could be fun - pick a significant game from your past and tell us why it's important to you. Doesn't have to be a good game, doesn't even have to be a game you like, just one that influenced you or shaped your taste in games in some way.
Me? I'm going to go for The Chaos Engine.
Growing up in a house with two brothers meant that games that were two-player were at a premium, and co-op games even more so. The Chaos Engine's co-op was frantic and brutal, and one mis-communication could leave one (or both of you) dead sharpish.
But mostly for me it was the vast, sprawling worlds - when everything else I'd played was straightforward and linear, CE's levels often had an obvious linear route (albeit with backtracking for keys and doors) alongside a multitude of alternate routes - ignore the first golden key and pick up the one behind it and you might be rewarded with an extra exit, or collecting keys in different orders could make teleporters send you to different destinations. Worlds 2 and 3 even had alternate routes that spanned several levels, meaning you could only access the special areas in later levels if you'd taken a certain route two levels previously. To this day I still don't know how to get to certain areas - tantalisingly close behind an unbreakable wall, yet utterly unfathomable. I love that kind of complexity that rewards repeat playthroughs, especially going through and experimenting with routes, or ways of approaching the landscape.
As a game I'm not sure how well it holds up these days - the archaic single-stick controls mean that a lot of artificial difficulty revolves around the in-ability to sidestep. Redoing it as a twin stick shooter would remove that, but whether it'd still feel like CE is another matter. Everything else around that would hold up I think - the inscrutable alternate paths and item usage is reflected in games like Dark Souls, and although it's been called visually dated I still love the art. Dan Malone is still around and being awesome while occasionally dabbling in games from what I hear.
Ok that's me done - what about you?