In all honesty, the best thing for that is a Blaster. That's not a putdown to sentinels; the thing you're asking for is simply what blasters are built for. They have longer range, they have higher target caps on their AoE, and without defenses there's actually a reason for them to stay far away. If you're staying at 40+ feet on a Sentinel, you're barely in range to hit things, and completely out of range for many AoE powers, especially cones. Even when you can hit things, you hit 6 instead of 10, or 10 instead of 16. And of course, between AT multipliers and Defiance, Blasters just do more damage overall.
But if you specifically want to do this with a Sentinel anyway, then AR, Fire, or Water would all be fine choices. Buffs and debuffs don't favor any set in particular.