First time I used this in Infected Normal, near the gate, I was very surprised when all the spheres and probes around me exploded. Fun!
Put it on your shuttle when doing the relevant missions. Use it when you get hit by the swarmers in Operation Gamma, and it just turns into a wall of explosions. I laughed aloud the first time I did that.
I'd have to pick the DS9 Bundle. I like to wander the corridors of my Defiant-R, touring the ship one last time before the next bout of respawns. Other than that, Doff space expansions and the Defiant itself.