In theory I feel that the dual magicka + stamina 'drain' poisons are best. I say 'in theory' because I'm yet to find an effective use of them over buffed enchantments.

For your second question - AFAIK jabs won't proc it, it needs to be light/heavy attack or a skill from the 'Weapon' skill lines

If you're weaving basic attacks in between your skills, you can still proc the poison. The magicka/stamina draining poisons are useful in PvP for shutting down your opponents abilities. Their main use in PvE would probably be damage (a combo of ravage health and gradual ravage health can do over 8k damage with a single activation).

The main limiting factor to poisons is that they take time to activate (20% chance per hit) and they take effect over time (usually lasting 3-8 seconds). This makes them very inefficient for any target that isn't going to be upright for more than 20-30 seconds. In most situations, normal weapon enchantments will be better.