Dark/Dark is a great tank at high levels and once built out, completely sponge. But unless you see it past 50 with enhancement sets and incarnates, it can be tricky to manage it's endurance consumptions as its protections are toggles. I have a 50 Dark/Dark/Dark Patron - it's pretty untouchable, can jump into anything and hold aggro, or play offensively and help - but not lead - team damage. HC tanks hit a lot but they aren't *really* the high damage dealers. Brutes can get up there, damage wise, but they come with a trade off (not as survivable as tanks generally speaking).
Rad/Rad can be a very good tank build.
Honestly the key to trying anything out in COH is to have inspirations ready to eat, that way any stat holes the ATs create can be smoothed over a bit in combat, help you get a sense for what the build can do.