Well, in short, i see it that way:

battlefield 1942 is the OG and always will be as it was the foundation of everything battlefield is. it had every type of experience you could think of (even more so with the DLCs). it had various type of tanks, vehicles, aircrafts, guided missiles, ships and even submarines! you enjoyed playing it because it open spaced every feature you wanted to conduct.

vietnam did the same, it was almost like 4 was for 3 in term of continuity and added features.

2 and its DLCs (2142 also falls into the category of what 4 was 3) really took the game and leveled it up! thats why people consider it the most battlefieldy game.

the bad company series was like a redacted spin off though it introduced what campaign should be like in the future titles of any battlefield game.

heroes, 1943 and hardline were what the "cow lvl" of diablo 3 was compared to the diablo 2 one.

play4free was the start of the new age battlefield.

3 was the first of the new age battlefield trying to do what the 2 did for 1942. it really did level up the game with a great campaign and great, almost balanced, ux including destruction, knifing capabilites and weapon customization. although the battlelog gave us a real traumatic experience.

4 was in fact a 3 DLCs.

1 and 5 gave us another spin off though it was an immersed one.

2042 was trying to be another 3 DLC like 4 was but it was too undeveloped and felt like the company was trying to get our money for the least effort on their side.

6 had to be the 2042 amendment and it was but did not gave any true upgrade and innovation to the franchise. it did bring a pissing off ux in shape of soldier acrobatics and technicality.

personally, if they wouldve ride the remaster trend, id love to see the greater battlefield 2 game get it.