Far Cry 2 was great on paper but a chore to actually play, when I got to the 2nd map I was like nope and quit since it was just frustrating to have checkpoints respawn so often like that and even though treasure chest hunting in 3D open world games was still really new (AC2 wasn't even out yet) and fun to me I couldn't abide the method for finding the diamonds, it became such a chore quickly.
Far Cry 3 came out and I was super hooked, hunting the specific animals for upgrades was a tad rote at times (and final island was where I quit again due to burnout) but fighting underwater animals was quite a new thing at times so it was so fun to live in that world. Hunting chests in caves was also super fun as each cave had a different design to it and felt like proper treasure hunting. Plus the hallucinogen missions were quite fun among other aspects.
Then Far Cry 4 improved the gameplay of 3 again a bit, made the main missions a bit more exciting (not much, the 2 "different" main characters you could follow were boring af and identical really), the main additions being the mini-copter and riding animals, which added some variety to the base-busting that was quite repetitive in 3. The treasure hunting was mostly pants in 4 though, all the caves were almost completely identical boxes. The drug missions were even better iirc and the special caves were well done, diving into cenotes and such. Also the landscape itself was more fun to traverse than 3 because you had the wingsuit from the start and so could go up the sides of the valley and then wingsuit back to the villages to finish quests or straight into a base or the like. I also found Pagin Min a more interesting character than Vaas (even if the actor played Vaas better) and the final battle was relatively fun.
Far Cry 5 is by far the best of them all imo, the melee from bats, shovels and such was mega fun, the treasures/bunker/stash puzzles were just the right length and difficulty making them really fun. The main characters were silly and fun. The music was immensely good, one of the best modern soundtracks I've heard. They finally added proper flying in helicopters and planes. The stealth combined all the stuff built up in 2-4 with some more changes. The skill tree was fun to fill out (finding booklets in the world to get skill points is excellent because then you progress the same even if you just ignore the missions entirely, well you can't ignore the main ones if you make territory progress anyway...) and the drugs/injections system was a great alternative if you didn't want to find the skill point booklets/do the stashes.
The map was always nicely varied with different little touches for each area (the drugged rivers in one was funny) and it was actually fun to fully explore them due to stashes and side mission collectibles (lighters and... playing cards maybe? I can't remember). I also liked doing up the towns and getting easy access to vehicles and guns, again an evolution from 3/4. Oh also the dare devil side missions were very funny and fun. The companions were also excellent fun to recruit and travel with, their AI was pants at times but it didn't matter when you had upgrades since they would respawn (or even heal themselves) quite quickly, that made playing totally solo more fun for me. I never played it coop but I'm sure thats even more fun since you could do some aerial + ground attacks and cause massive mayhem.
The only issue was the base busting was getting really rote at this point and they didn't really mix it up enough to keep it exciting for the whole game (or even for one territory, of which their were 3 or 4), adding annoying armored bullet sponge enemies to stop you taking over the base in 10 seconds unless you abused specific weapons/mechanics.
All the above (and more I'm forgetting no doubt) added up to making FC5 the best one by far for me... and then FC5: New Dawn came out and it was total trash with the bullet sponge bullshit, I turned it off after 60 mins of playing and never went back.
Far Cry 6 was more back towards the style of 5, but the combo of the map being far too huge, the online MGSV ripoff (but not as good) "send your guys out on a mission for XP/resources" element was bad and a chore and the New Dawn bullet bullshit was even more annoying than in that game somehow.
Far Cry 7 needs to be an actual evolution of 5 and get back to the open world treasure puzzles combined with good narrative content. I have doubts they will go back to a smaller map though, they want these games to take 100s of hours to complete and I just can't be bothered with that after nearly 20 years of these type of games, I enjoy living in a world and scouring the whole map, when the map is so giant and repetitive I just get burned out and the rote narrative elements aren't something than can keep me interesting enough to continue playing. Those are things I put up with because the open world exploration is so fun.