Juggs/Guards are very rewarding now, and I'd easily put them forward as the best tanks. They have the greatest cooldowns, and amazing ranged threat generation. In flashpoints and select operations, a Jugg/Guard can tank entire trash pulls by getting initial aggro and popping saber reflect immediately.
They also have the strongest cooldowns of any tank class, even eclipsing shadows in fights with tech/force attack mechanics in all areas but up time (Shadows/Assassins can get their immunity cooldown for these attacks back much quicker).
In PvP I'd say that if you're committed to keeping your healers alive, hands down go with Guardian. Guardian leap is an amazing tool not only for getting control over one person's bursty DPS (if boss facing isn't an issue), but also for ensuring that you won't be parted long from the healer you're guarding in Warzones. But as tanks go, I'd say that they under-perform in raw damage output in PvP. I haven't seen any guardians successfully rock DPS gear in a tanking specialization like Shadows or Vanguards can.
As for Vanguards/Powertechs, only thing I'd say they have over guardians is against tech/force attack heavy encounters. Their emphasis on getting shield chance and absorb high makes them shine in these circumstances. They also have notable utility with their 'pull,' like a Shadow, as well as the ability to ignore certain root/slow effects due to their new 'hold the line' ability, which is very useful in select operation encounters.
But ultimately you need to decide what you'll be doing with this tank. IF you are going to be operations, I'd say stay clear of Shadows, and look at the crew you'll be doing operations with. It is generally a good idea to stagger your classes, and not have two of the same class in the same role. Vanguards and Guardians shine in their own ways, and have skills that complement eachother, but this won't really come until play until you're at the bleeding edge of progression raiding.
PS: One note about Guardian tanking. Outside of saber reflect, attaining initial AoE aggro and maintaining it in the first few moments takes practice and effort if you don't want to blow your AoE taunt. Shadows and Vanguards have a much easier time with very simple and powerful AoE threat building tools that is less dependent on the placement of enemies. With Guardians, to cement your AoE threat, you need to be able to execute a brief checklist.
- Put the armor debuff on the target via Sundering Strike or Force Sweep/Smash
- Make sure you are targeting a target close enough to the four other targets you want to hit before striking. This can be tricky if you don't have initial aggro.
- Use Guardian Slash
Vanguards just have to use their cone attack or their targetable AoE attack. Shadows do not have the first step in the Guardian's checklist. Both also have a Personal Body AoEs that is not a strange cone that multiple adds like to dance to the edges of, and Guardians will have to backpedal in small circles if they want to continue hitting all five baddies with cyclone Slash. The other classes have an easier time wrangling enemies, but Guardians, with practice, keep pace.