I'm almost sure this has been discussed in the past... =D
Anyway, I'm not going to waste my time in posting exact classes for you to use. I used to believe in those sort of things, but then I came to the realization that all players do better with different combinations. There is no "best" weapon for all players. My advice would be to stick with a couple of combinations. Put some thought into those classes, and really decide what kind of player you want to be.
The next step is the harder one, and the one many people skip. Stick with it! At first, you might suck with that class, and yes, your K/D ratio might go down even more. If you stick with those couple of classes, though, you'll find that over time you'll get better and better with them. That's what I've come to learn, at least. Hopefully you can find a method that works best for you.