Go with a shadowknight. Go with a DPS build. This means going down the INT AA/AP line. I personally like the STA line also. I recommend equiping them with legendary player made devout plate. It will boost your STR, INT, & STA. Go with a good one handed weapon, a shield, and offensive stance. You'll want to fill your charm and jewelry slots with gear that increases your AC/MIT and INT. 75-80% of our damage comes from spells. Our power poll comes from STR & INT. Lastly, insure that your spells are upgraded as much as possible. Upgrade your stances, ward, and lifetaps to adept III or master, at least.
This build will allow you to solo like a god. You will also be able to tank well if needed. Where you'll truly shine in a raid, though, is as DPS. With this kind of build, you'll be able to get into any raid. You will be putting out some serious DPS.
Even with all of this, you still need to know how to play the class well for people to pick you up for raids and such. Pick the class that suits your playing style the best, and then learn to play that class well.
Somebody suggested making one of each tank class. That's a good suggestion. You can make a new tune and level it to level 20 in one good day of grinding. Within a week, you would have a really good assessment of which one would fit your playing style best. For me, it's the shadowknight, hands down.