There are five sets of operatives, The French GIGN, The British SAS, The Russian Spetnaz, American FBI and my favourite GSG 9 from Germany. Why did I reccomend them as a starting point? Well, first off they have the most heavily clad attacker in Blitz. Blitz uses a riot shirld, which limits him to a pistol only, but he can take several hits before taking damage if used correctly. Add to his shield the ability to flash and blind enemies making them easier to kill for you or your team mates and you have an almost unstoppable killing machine.
Pair him with GSG 9's aggressive lady and they become inseperable in combat (unless the SAS operative Thatcher is in play, in which case she should be paired with him), she can scan for explosives and enemy gadgets through walls, which is hady to help the team plan for an area of attack. She has the lowest armour rating though, so try to act as more of a supporter if you're playing as her.
In all, I'd personally recommend them as you're first port of call. Mix them with the SAS operatvies and you can almost taste victory, for the fifth operative use Glaz of the Russian operatives. He can pick people off with a sniper while Blitz keeps them busy.