I pretty much go with the "vanilla, lore friendly, default class" during my first playthrough (which i speed run) and then go with the class that is completely different than the first class in my next slow, serious playthrough.
In ME3 and ME2 i always pick the soldier class on my first playthrough. I picked the soldier class because i want to get a good feel of the shooting mechanics and guns in the game, In ME2 you really dont have that much freedom to spec you character the way you want like in ME3. the skill tree is too linear and you only have the choices on the last level of the skill, which i think was too late. As a soldier i wanted to be the bad ass Rambo who just runs and shoots, so for all the skills i went for anything that does more weapon and melee damage. I ignore anything that gives the squad bonuses and stuff. For my more serious playthrough i usually go with sentinel or adept. Sentinel because its a really strong class with its mix of Biotic and Tech powers and Adept because its completely different from the soldier class and its also the most fun in my opinion. For Sentinel i go for defense, squad bonuses radius increase and capacity increase (for grenades), i rarely go to skills that increases damage because i dont think thats what a sentinel does. I basically just turtle and just provide support when ever i can. whether its taking down barriers or shields, providing firesupport with my high radius grenades to take out large groups or to be the guy who takes the hits to give my teamates a chance to flank around. For adept i pretty much do what i do with the sentinel class which is supporting teammates, i go for the skills that increases the radius of the move and the recharge speed.
Thanks for looking and i hope i win! :D