For me, Far Cry 2's Africa was the most atmospheric and interesting setting.
But in terms of gameplay and structure the games just keep getting better. Far Cry 4 is absolutely fantastic - just a few niggles shy of perfection, really.
I do think the games are a little too animation-bound. FC3 was even worse, as you had to watch an animation every time you looted a body, but thankfully 4 allows you to turn that off. But stuff like the eagles are just idiotic - you get locked up in an animation watching the eagle swoop in and attack when it's about a mile away from you.
Those eagles would've been really neat if they attacked you rarely - just a couple times in the course of a playthrough, rather than constantly. That would've been a spectacle, rather than a pain in the arse. And constantly hearing terrified natives yelling "aargh! Eagle!" then firing their weapons in vain gets really old and breaks immersion. Rest of the wildlife is pretty neat though, although it's kindof dumb how an entire assault rifle clip is needed to kill even medium size animals. When you unlock the Bull its a two-shot kill even for rhinos and bears, but new players may not know that and consequently the wildlife could be frustrating simply because it comes out of nowhere and you feel somewhat defenseless. Also, I've been stunlocked to death by yaks and rhinos before - the canned animation knocks you down, robbing you of control. You get back up and are immediately knocked down again by the goring animal, having never regained control. Repeat until dead. Didn't happen often but when it did it made me curse Ubisoft for being such control freaks, always on the lookout for some petty opportunity to remove your agency.
'Press Y to extinguish fire', and other healing animations are just tedious. I want the next Far Cry to get out of my way more.
However adding the Buzzer was a masterstroke, as it takes away basically any traversal complaints someone might have. Now you can use the roads (and almost certainly get attacked en-route, a'la Far Cry 2), or you can go offroad, or simply fly. In a game where traversing is your main activity, giving players this amount of freedom just makes more sense than you suspect Ubisoft are capable of. Very impressed.
EDIT: but fucking hell Ubisoft can you give the drug sequences a rest? Between 3 and 4 we've had enough of them. Why bother unlocking a ton of weapons if you're going to insist on the bow in some tripped out bullshit segment? There's far too many of them, and no, Ubisoft, your storytelling doesn't warrant the regular eradication of player agency like this.