For my opinion, BF4>BF3>BFBC2>BF1>(BFV ( I don't even want to buy it)).
First, this is a great game to play thanks to the quality of the graphics. There is a real atmosphere that we can feel.
I also liked the new and interesting mode : Operation, a single round that is played on several maps with several assaults. It permit to change instead of always playing conquest.
The arrival of mastodons during the game is also nice to reverse the round and are fun to play.
The maps looks also nice and I liked to be able to interact with, like in BF4.
But I prefer modern war. That's all.
Each one it's point of view on this question, because the historic context and atmosphere is very important while playing for the immersion.
So I hope that BF6 will be a modern one, not futuristic, not historic, just current. I want a back to basics : the current quality of graphics + BF3/4 gameplay and why not some ideas from BF1 + a campaign like BFBC2 with real charismatics characters <3.