Everyone is going to have a different opinion. Myself I would go with the the following order.
1) Assault, once you unlock the paddles...its point raping time and you can level fast. 100 points per Revive
2) Support....I'm not real fond with most the guns in this class but you can ride the resupply point pony pretty good.
3) Support is my favorite class once everything was unlocked, good guns and you get to shoot rockets...
4) Recon is the worst in my opinion for first timers... its a whole seperate beast from COD sniping. You can get great points with long shots but they are few and far between.
Hit me up tonight if your playing BF3........be my intervention and keep me from playing BLOPS2...