As broad as that question is, somehow, we all seem to know exactly what's being asked. We know it's a question about the aesthetic of that age and the attitude that gave birth to the style of fashion and artistic expression we saw.
The 90s was the perfect marriage of technological convenience and individual freedom. The rules of Western society had not yet caught up to its newfound power.
The Internet Was Actually Dangerous
Though 'dial-up' was far more prevalent in the 90s, at any moment you could stumble across content that threatened to turn your world upside down. You were just as likely to unwittingly end up in a forum filled with images of cartel killings as you are, today, to close a pop up ad for an online casino. Genuine classified documents could be found via a mere google search, along with the contact details of retail hitmen. What is now the dark web was just the world wide web. As Tom Noonan's Kelso, in Heat (1995, Michael Mann), said, "This stuff just flies through the air. They send this information out and it's just beamed out all over the place."
Your Money Went Further
Everything, from the cost of a pint to movie tickets, even weekly grocery shopping, was easier. Life's taxes were lower. You need only be orderly with your less-than-extraordinary paycheque to afford a 1 bedroom apartment - located where things actually happened. It wasn't too much to ask to live comfortably where everyone else of working age was. Today you need a side hustle just to keep your head above water.
Food Tasted Better
The FDA, and other three-lettered party-poopers, hadn't yet regulated the taste out of everything, so food & beverage competed with salt and sugar. With no regard for the long term health of their customer base, they'd pile the salt or sugar into their products to "even the odds".
But on a serious note... remember how sweet tomatoes used to be? I'm here to tell you even avocados used to taste sweeter. Yes, there is, in fact, supposed to be sweetness within the taste profile of avocados.
The Future Still Had Flying Cars
That's not to say we've collectively given up on that future... but we are now far more convinced The Apocalypse will arrive before everyone, essentially, becomes a pilot. In the 90s we thought we'd be on Mars by 2020. Today we're facing an energy/food crisis poised to set any form of technological progress back to Genesis. We're preparing for stagnation as opposed to the singularity, thinking ourselves lucky we still have electricity.
The Best Decade for Music
The internet had not yet stollen the show, and the old model of business existed parallel to modern production techniques. The result was albums like the Verve's Urban Hymns (1997), or almost all of Oasis' singles. Hybrid's Wide Angle (1999) pioneered orchestral electronica. Yep. Imagine breakbeat by way of Hans Zimmer. Popular music retained its classical nature. Melody was magnified by new production technologies, not exchanged out altogether - as appears to be the case today. I mean we could not have imagined mumble rap back then.
The Best Decade for Movies
The more problems we solve in reality the less contemporary fiction has to work with - in regards to manufacturing high stakes. Action heroes don't have the same excuses to get medieval on a mother-effah, which is why almost all of Daniel Craig's James Bond films where obliged to pose the question, "Do we even need you?" Of course, both the film and the audience had already made up their minds but it was an agreement to settle for a lessor experience. To placated, knowingly. We knew the events of Heat couldn't go down like they did in 1995 but we appreciated The Town (2010, Ben Affleck) for trying. But The Matrix (1999, The Wachowskis), despite Lana's efforts, remains a 'no-go'. The same can be said for Fight Club (1998, David Fincher). A film about a character numbed by his own job security, and zombified by his ability to consume endless, would be completely out of touch with today's gig economy.