Since the nerf I'm pretty much done with Omega and go for Beta 3 all the time. It has a 30 second cooldown and will be active for 10, good enough and twice as fast as Omega. It's quite impressive together with Sensor Scan on a sci/escort and even without any other damage buff my Fleet Escort shreds through STF Borg really fast.
Delta would be your choice when you get aggro from more than 1 enemy on a regular basis, for example on cruisers. Too bad they can't really take higher levels of Delta.
Second. These are activatable powers within your boff tray. When you sit a Bridge Officer, or boff into a star ship, they have a set number of skills they can use in that seat. Each higher tier of skill gets more progressively awesome, but ships usually only have one Commander Seat. So you have to pick and choose which skills your crew, and thus your ship have.
However, each and everyone of these powers are activatable and must be selected for the power to have any effect. These effects will work over a short time and then they will stop and will go into a short cooldown before you are able to use them again. This goes for all boff powers, including attack patterns.
The only things that gives you passive (nonactivatble) buffs are gear and gear sets, consoles (which some of the Zen and special consoles are active as well), accolades, certain doff missions rewards, as well as your traits on your character and crew. But Passives are gradual and generally don't drastically impact gameplay unless you min/max the system.