I started playing BF with 1942, enjoyed it so very much.
They need to make a new WW2 Battlefield game. Or even just re-release 1942 with upgraded graphics and environment. I'd buy it.
I played BF2 & BF Nam a little but didn't much enjoy either one much.
BF2 couldn't hold my attention and Nam okay but not that great.
Played very little of 2142 but was fun. Loved the futuristic feel and atmosphere of it all. Battlefield in the future is pretty bad ass.
They need to do another futuristic BF (2143 please) soon.
BFBC2 was fun, loved the destructible environment. (played on the eGO servers there)
Didn't play 1943, Heroes, or Play4Free so no opinion on them.
1943 was just a graphically upgraded version of 1942 pacific maps with a few changes I believe.
Played the hell out of BF3 and loved very minute of it. Best map ever made was Seine Crossing. I could totally own tanks there with c4 IEDs & ambushes. (If you ever played on the old DoW 24/7 Seine server you'd know lol)
BF3 has some of my favorite maps and is the BF game that got me really hooked on the franchise.
BF4 is good but it's missing some things, namely destructible environment like in BFBC2 and BF3. I'd say BF4 is very good but isn't fantastic.
Hardline is meh. It feels like BF gone CoD. Faster paced than all other BFs. Not as many guns. Maps are not that great. Cars handle just as bad as ever. Lack of huge range of guns.
BUT by far the worst thing about it in my opinion is that you have to "buy" each item, not unlock it. I can't stand games that do that. I like the way of leveling up and unlocking guns or having set load outs for each class, not earning money and then spending it on things.
For me BF3 and BF4 are my favorites.
If you want to get really in to the Battlefield franchise fully I recommend starting off first with BF1942 and BF3.