Honestly if we talk about the mainstream alone than I have no doubt the most beloved Fallout game was actually 4.
The divisiveness of the that game only exists in the hardcore bubble I highly suspect. Outside that it was reviewed positively and did ridiculous sales numbers.
Hm, I'd disagree there. I think Fallout 3 gets a credit for being people's first Western RPG that, oddly enough, games like Oblivion don't really get anymore (probably because it was replaced so quickly by a "mainline" game and sold slightly less). Fallout 4's critic reviews are quite a bit lower than Fallout 3's too, and I think that Fallout 4 was one of the first times where the casual fanbase actually complained about "casualization" in a mainstream game.
Then again, casual fans are still fans and not just casuals in general. In that respect it's a hard comparison anyways, Fallout 4 sold so much and it's audience is so expanded compared to 3's!
Fair points, and I get where you're coming from
Though about critic scores, despite F4 scoring a few points lower, 87/88 is nothing to scoff at. Just recently Spiderman got an 87. Farcry 5 got 81. These are games that casuals (or casual fans, as you say), absolutely ate up.
On Xbox's "most played" charts , the game is still regularly in there, despite being three year old single player game (for comparison it's above sea of thieves right now which is gaas game with 5 million players, and designed specifically to keep players playing for a long time, as Fallout 4 isnt. Gears 4 and Halo 5 are lower too). The game appears on and off in UK's sales charts too. The game of the year edition that came out summer of last year still retails at the full fat 60 bucks after a year and a half.
When a single player game has this much legs, I think we can conclude that a lot of people really like the game.
Though this is just my viewpoint. I think you have a good argument too.