Fallout 2 was my favourite game in the series for 20 years, until I replayed first two games last year. Now somehow, oddly, I prefer Fallout 1. Atmosphere is more serious, it has more bleak tone than the second one (or any other entry in the series). Still - both are there at the top of the best crpg of all times.
The rest I liked to some extent, but it just felt so off the mark. They're not BAD as some religious fans make them out to be, it's just different game design philosophy. Also Bethesda never completely understood what Fallout is really about, they just mish-mashed parts of the universe in their games they thought were cool.
I also understand fans that started with the 3rd game in the series and their love for it. I'm not gonna lie - I was hugely disappointed with third game when it came out and reaction was something along the lines - lookhowtheymassacredmyboy.gif. First two are now pretty archaic and hard to get into, but pretty rewarding in the end. Remaster or full on remakes would be awesome, I would even take first person version of that. Shame though, that some of the themes would probably be cut cause of the current oversensitive climate (you can't even make post apocalyptic game anymore with all the horrors that humankind is capable of, everything would probably be watered down).
So, for me:
1. Fallout
2. Fallout 2
3. Fallout: New Vegas
4. Fallout 3
5. Fallout 4
6. Fallout: Tactics (didn't enjoy it back in the day)
Didn't try abominations like FO and Fallout: Brotherhood of Steel.