The first Smash Bros. came out for the Nintendo 64 in 1999. The latest Smash Bros. came out for the Wii in 2008. So of course, the differences in looks are rather dramatic.
This video from YouTuber Master0fHyrule focuses on how the original twelve Smash Bros. characters' taunts have evolved over the years. However, it's also a very effective comparison between the three games' graphics—especially the character models. It's quite the trip across gaming history:
I love Super Smash Bros, such a fun game to play with friends. It's probably just me, but I like the first Smash Bros best. Not sure what it is about Melee and Brawl that don't give me the same enjoyment as the original.
Either way they're all great games to pick up and enjoy.
My order of preference is Melee, Project M, Brawl, Original.
I have played both demos for the new 3DS and Wii-U Smash coming out later this year, but not nearly enough to determine how much I like it. Plus, it's not the final product anyway.