I don't have a particular favourite power card, but I'm definitely fond of the traps. They especially help when you're down on your last shred of health and your only option is to run around and hope that boss you're fighting accidentally steps on one.
As for epics, I'd say Relentless/Burst Fire x2. Yes, there's only a 35%-ish chance it'll activate, but if you're on a good roll of luck, you can deal a pretty good amount of damage in one round.
I always try to train Relentless/Burst Fire/Mojo Echo on my companions when given the opportunity. I also make sure that those are trained high up in Accuracy to ensure that they hit more often.
Pairing Relentless with First Strike on some of my Buccaneer/Swashbuckler companions has often times seen the enemy go down from full health to dead in one round. Especially if it's triggering the mobs' own epics like Vengeance Strike This happened just last night while running my Buccaneer through the Labyrinth. I had my newly acquired Eagle Hoplite, Zelos Maximus (LOVE the name btw) and he just killed one of those Buccaneer class Ophidians by himself in one turn!
Of course these options aren't always available so I have to say that Riposte is up there at the top, too. With this epic I make sure that my companions are high on the Dodgy side.
When I first started, though, Bladestorm was my favorite cause it would enable me to kill a minion who was low on health then be able to have the chance to start whomping on a second! As I've progressed through the game and worked more with other people, I've realized this isn't always feasible because my crew doesn't always get to go first. Which means that those I've targeted in hopes of this epic activating, won't because the crew that went first almost always targets/kills them before mine do.
However, the Musketeer equivalent, Double Tap, is still excellent. Because they don't need to be standing next to, just in range of, then they have more choices on who their second (or third if trained x2) target will be.
edited: changed are to aren't and added Double Tap