Dark Armor - This is a resistance based set that features exotic protections against end drain and psionic attack. The primary weakness is to energy attacks. It has a powerful self heal (dark regen) that requires a to-hit check. This set is very endurance heavy.
Electric Armor - This is an excellent resistance based set with some great utility powers. Lightning reflexes increases the recharge times of all your powers, and power sink is a top notch endurance drain.
Energy Aura - This defense set is very similar to the brute set of the same name. It is a defensive set with a few passive resistances as well. It has interesting utility powers in the form of stealth, energy drain (drains endurance), and possibly the best "Tier 9" defensive power: Overload, which raises max hp as well as all defenses.
Ninjitsu - This is by far the most unique of the stalker secondaries. It is a defense based set, but fortunately it is damage typed defense rather than positional. It has click based status protection which some people prefer for PvP.
Regeneration - This set mitigates damage by regenerating it back (hopefully before you die). This is the "Wolverine" like defense set.
Super Reflexes - This is a defense based set. The problem with this set is the defenses are not damage type based, which means they do not stack well with other defense bonuses.
Willpower - The newest of the defensive sets, this is a combination of defense, resistance, and regeneration. It is an excellent set - especially when combined with the Fighting Power Pool.