A couple of things to consider:
Sentinels will take a long time before they really start to shine. (Usually around the higher 20s, early 30s.) In the lower levels, their damage is relatively low, their ability to survive is simply laughable, and if you ever want to PVP during your way along the game; you're completely shafted until mid-game. HOWEVER, given that you like to deal damage and JUST deal damage, the sentinel gives you a couple of really neat ways to do that, along with providing some decent buffs to your party in both PVP and PVE. Just be wary of this class, as they have a numerous amount of keybindings. (This part is really important. You're only in medium armor; you MUST remember your defensive cooldowns or you are nothing more than dual wielding cannon fodder.)
The Guardian, on the other hand, picks up around level 14 if you plan on defense specializing, and at that level, if you PVP, you aren't quite as shafted. Yes, being melee, you'll be CCd quite often, but you'll begin racking up those medals and guardian points so long as you remember to guard people. Now if you don't like guarding people, your damage will never be "chart topping" until later on, when you get more of your utility abilities.
As far as shadows go, they definitely have an advantage in PVE and PVP in stealth, as you save a lot of time / deaths by sneaky sneaking around. However, I don't usually see them chart topping. That's not to say they can't do great damage, accomplish objectives, and support the party; they just aren't your raw numbers guys.
If you're wanting classes that do lots of damage and avoid major tanking/healing roles, you'll probably want a ranged class of some variety. I myself am a huge fan, player, and supporter of melee classes, but they are the much more difficult to play in terms of situation awareness, positioning, and surviving.