Well, you could argue that it still is, or at least that it is in the top 5! There is not a single thing you could change about Tetris to improve it.
Yes, it's far simpler than the likes of Half Life, Bioshock, OoT, RE4, etc, but it is almost objectively perfect in terms of what it sets out to do, what it achieves and how there is almost no room for improvement, even now. They did try to improve it many times and each iteration came nowhere close to the original.
But I do take your point. Games move on and many titles which were absolute gems at their time of release don't always hold up well by today's standards (but maybe we've just become accustomed to better controls and graphics, open-worlds, online play, etc?). I'm a retro collector and use to occasionally write features for websites and magazines; often I'd find that going back to play some of the old games for a critical perspective ended up killing the nostalgic feelings I had for them when I realised they weren't as good as I had remembered (Altered Beast is the prime example - great at the time, woeful by today's standards).