Ok, of course this is an opinion. To me, if the the owners of the series say its canon, it is. I really hate how fans think they are entitled to decide what is and is not canon. I enjoyed Fallout 3, the story idea was good though the execution of the story and voice acting was medicore, i admit. But its canon, semi-canon atleast, cause New Vegas has confirmed it with dialog with ED-E (more so in Lonesome Road) for the CW Enclave, and Veronica for the CW Brotherhood and Outcasts. And New Vegas was made by Obsidian, former members of Black Isles, and not be Bethesda. Bethesda just published it. And im sure this is what they are gonna do for future Fallout games, have Obsidian make the game, so they can focus on their Elder Scrolls series.
Anyway, in gameplay terms you cant really compare it to Fallout 1 and 2 because 3 and New Vegas are FPS/3PS RPG while 1 and 2 are isometric turn taking RPG.
To me, I like Fallout 2 better than 1 simply because you can play past the ending in Fallout 2 (they both are fun regardless), while I like New Vegas more than Fallout 3 because of the reputation system and the more crafting options. Im ok with not playing past the end for New Vegas, because of the different endings and paths you can take, and simply took Fallout 3 formula and made it better. Both are pretty fun Fallout games.
Though I do admit Fallout 3 should have been called "Fallout: Capital Wastes" and Fallout: New Vegas be called "Fallout 3" because New Vegas is closer to the location of the first 2 games.
Anyway, if you like isometric RPGS, play Fallout 1 and/or 2, if you like FPS/3PS RPG, play 3 and/or New Vegas.