An ArmA thread.. it made me make an account and stop the lurking. I'm a real ArmA fanatic (it's even an understatement really, here is my arma 3 alpha 1st impression to get an idea
https://www.youtube.com/watch?v=mNDLLIjuGac ) so I have to contribute
.
If we are talking about vanilla single player, then yes Operation Flashpoint (later renamed ArmA: Cold War Crisis with patch 1.99) it's the best one. If you get a proper up-to-date version you should have 3 awesome campaigns to play: 1 for each faction that it's involved in the conflict. Like all ArmA games, OFP had an awesome modding scene (I had 150 gigs of mod, nerdly organized, but lost it all in a failed linux installation ), so obviously there also some epic user made campaigns for you to play, like Nominesine's Blue Skies which is considered to be AT LEAST on the same level of quality with the vanilla campaigns. Another user made campaign that is highly praised in the community is Revolt, but sadly I don't remember it though, what I remember is playing the shit out of a campaign (don't remember then name) that came with the user made Tonali Island and also try the campaign that came with FDF mod. The problem with user made campaigns though is that most of them are dependent on other unit mods/maps etc and they can be a pain the ass to find (some sites that hosted OFP mods are down).
Also you might have some problems running mods with 1.99, because of the executable names, a problem easy to fix, BUT there might be other problems as well. I'm not very informed here because I preferred to play 1.98, since I didn't care much for the name change. Most mods were made for 1.98, since 1.99 came extremely late, 99% of the mod scene was dead when that patch came and you can't downgrade your 1.99 to 1.98 (this is random shit I remember, but you should google stuff like this yourself to be certain). I would try to own 1.98 and 1.99 if I were you. There are reasons to own 1.99, like the mod binmod for example that is exclusive for 1.99 and brings a lot of cool shit to the table, like real iron sights something that most thought it was impossible to do with this engine.
The big problem is that for you to enjoy the game you must tolerate the graphics (
https://www.youtube.com/watch?v=F96w7p-Z1KY ), it didn't age well. For me or other people who loved the game back in the day it's not a big problem, but for someone who it's going to play it for the first time in 2015... I don't know what to say. If you can't get past this, you can use some mods, replace the sky, replace the weapons/units (make sure it doesn't fuck with the AI though so that it doesn't affect the campaigns), HD variants of the islands, Kegetys DXDLL. Another thing from what I remember the view distance affects the AI and the vanilla campaigns were designed to be played with 600m or 800m view distance (memory again, google to be sure).
The truth is now this series all about the multiplayer, very few care about the singleplayer. IMO to properly enjoy ArmA and to witness this series full potential you need to get ArmA 3 and join a big milsim clan that properly simulate military operations (ex 16aa). Stay away from public servers, but if you dont want to join a clan google unitedoperations, they function as a community who tries to play properly. They don't always succeed, but it's still a decent ArmA experience. If you are interested in this subject:
https://www.youtube.com/watch?v=OzZqpuJMC38 .
But then again this is what I like about this series, since it's more of a sandbox experience ArmA means a lot of difference things to different people, I enjoy realistic military operations, but you might not enjoy them. Maybe you like stuff like this
https://www.youtube.com/watch?v=Jpljei6wrgw .
In theory, Operation Flashpoint -> single player, ArmA3 -> multiplayer, BUT there could be some astonishing user made campaigns/missions for Arma1/2/3, and knowing the modding community that this series has, there is a 99.9% they exist, so you should be good with just ArmA3 for single player and multiplayer.
P.S.: But buy OFP as well even if you can stomach the way it looks, you are not buying just an ugly game, you are being a piece of history.