Arma was one of the first games I got when I built a PC, it's all personal preference.
I first started playing Altis Life RolePlaying game-mode I found a server and had no purpose so I became a cop and That was really enjoyable it helped me learn the gunfight mechanics and so on. Altis life from my perspective is to have fun and its very emotional if you play with the same community daily ex: if the same gang of friends tends to attack you all day, you learn to avoid/despise them. Also you meet some really unique people on there I made over 80% of my friends on there.
As I was playing altis life I got into some wasteland and wasn't enjoying vanilla wasteland so I downloaded some mods and play Chernarus wasteland (mods can be found in this launcher https://a3cdn.com/ hit me up if you get into it and need help setting it up.) This game-mode was fun cause you had some objectives to choose from. You can go for crate drop missons get OP guns and enjoy them, or fill a truck with guns sell them and then buy a sniper or something you'd like to try. And with the mods they add common weapons and military vehicles such as the M4, AK-47 and fal, Vehicles like Humvee's and Apaches.
I had recently just got into sector control or mainly known as King Of The Hill. This game-mode like altis life requires no mods its hosted by the same group that hosts Chernarus wasteland so some things are alike but this game-mode's obj is to control a city and till you reach 100 points there are 3 different teams. And I apologize for my laziness please see here (http://armakoth.com/) its all described there.
Some I sort of tried but didn't get too into was A3L servers. Those are the servers that are modded altis life servers with real life vehicles and more strict roleplay I went into Critical gaming which can be found here (http://forums.criticalgaming.org/) if want to try a server like this, sign up and application is required.
Well that's my 2 cents just find what you enjoy .
Edit: Someone should show me how to use hyperlinks on this website