To this day, I still consider Warcraft III to be the single most fun RTS game I've ever played.
If you're like me and used to be fully addicted to this game, the blast of nostalgia hits you hard and doesn't let up. The opening cinematic of Warcraft 3 vanilla is a masterpiece and gave me chills the first time I saw it. I remember when I was younger I used to watch the preview trailers for WC3 over and over before it came out. I had always been a fan of Warcraft 2 and Starcraft when I was a kid.
This game just has so many alluring aspects. The art is beautiful (The graphics won't blow you away by today's standards, but I still love it) and the concept art for the game is simply incredible. The bright colors and unique characters leave a memorable impression on you.
The four different races (Five if you include the non playable Naga, but I'm sure there are custom maps that allow you to play as them) are all unique and have a powerful, varied array of units. Each race also has four of it's own unique heroes, not to mention the mercenary heroes and units for hire on certain melee maps.
Heroes, in my opinion, are a big part of what made this game so damn fun. As much as I loved the original Starcraft games, the lack of heroes in Starcraft 2 made the game boring. There is nothing more exciting than using your high level Blademaster, decked out with powerful items, to Wind Walk and snipe off enemy workers or heroes. The possibilities are endless. And while the game might not be the most perfectly balanced because of them, heroes just add an awesome little touch of RPG to the RTS genre.
The campaigns are fun and full of side quests and cutscenes. The comprehensive map editor allows an endless stream of unique maps which helped spawn Dota and subsequently League of Legends and Dota 2. The fact that this game was powerful enough to spawn an entirely new genre speaks for itself.
The multiplayer on Battle.net was always the highlight of the game for me. I spent thousands of hours creeping, rushing, pwning newbs and getting pwnd online.
Ultimately, Warcraft III is one of the most classic, colorful, fun, exciting and enduring games of our generation.