I tend to go reactive or degen the most as you get additional damage AND some bonus damage in the form of -MaxHP or -res. As tidge says, Reactive's -res tends to be better for clearing crowds of scrubs and Degenerative is better for big sacks of HP like AVs and GMs.
I have added Diamagnetic to dark blast characters and especially defenders and corrupters that also do some -regen so that I can stack both effects to help the team and myself.
I like Spectral on controllers. It adds immobilization, which triggers extra containment damage, and negative damage, which isn't wildly resisted. It's not picked as often as the three above so that's beneficial for stacking a variety of effects, especially on Leagues. And I usually pair that with Control Core Hybrid, which grants an extra mag of control and bonus psi damage to controlled enemies.
Preemptive I have used on characters that I want to be sappers. I tried Gravitic on a gravity controller just based on theme and it really did slow critters down to a crawl. I could also see using the radial branch on a /cold or /poison to stack on top of Benumb and Weaken.
I have tried cognitive several times and am always disappointed. It rarely if ever confuses anything and the psi damage is less than other DoTs. If you use it to stay on a psi theme you are waaaaaay better off going with any other DoT as everything that resists psi resists it A LOT (like all robots and there are lots of bots at 50).
I don't think I have ever used Paralytic. The only use I have for -defense is to slot an Achilles - res proc and Interface can't do that. Well, that and Paragon Protectors going godmode -- but you are better off killing them fast than hoping you got enough -def on them. I could, perhaps, see combining the -dam with the - dam in Void Judgment. But at level 50, especially on teams, I much prefer more damage to more survivability.