T2: I used a Raptor as an engineer because Klingons badly needed a tank to uncloak first and survive the Fed hate.
T3: I used a battlecruiser for the very same reason. It's a really good ship. Fed escorts died quickly and other Fed ships weren't more agile than me.
I used 2x 180° cannons, a 90° beam (beam overload) and a photon torpedo (high yield 1+2) up front, with turrets only in back. I could have used rapid fire, but I prefer HY2.
T4: Battlecruiser again, similar setup.
T5: (only experience as a fed)
Save for T1 (BoP only) and T2 (no BC yet) I think BoP and BC is the way to go.
You cannot focus fire properly with a carrier, the CV is a no-tactics-no-skill ship.
A mix of 1-3 BC and 2-4 BoP makes a lot of sense. the BC serves as tank (first to uncloak) and the BoPs serve as swiss knifes with their universal BO slots (lots of science trickery)
The Raptor is just an escort with a cloaking device - quite pointless.
The optimal Fed team layout is 3 cruisers and 2 science ships.
The cruisers should emphasize turrets with only one torpedo tube and the much more agile science ships should mix beams, two quantum torp tubes (with HY) and rear turrets.
That way the cruisers could instantly proceed with tearing down shields without wasting time with slow turning and the science ships could use advanced science trickery plus finish off enemies quickly with torpedoes. The science ships would be primary target(s), though.
An alternative is a 5 cruiser tank group setup with excessive survivability.
Personally, I like the Negh'Var, course I went engi so it's more effective for me IMO.
I run a Dual Beam, Dual Cannon, Dual Hvy Cannon, and Torp up front and 3 Turrets and mines in back. BoP-ish (not quite) firepower with excellent survivability. The maneuverability is good enough that i can keep my fore weps in line about 80% of the time, although sometimes i have to burn Evasive to keep it there. I can run in DPS mode with max wep pwr or i can go into tank mode (max shield pwr) in an instant. So while enemies target the weaker shields and hulls of BoP's and Raptors i can heal/dps. And if they drop them i can quickly switch to tanking and allow my teammates to respawn and come in while the enemy is focused on my ship. Very rarely can Klingons come away with a victory with less than 2 Neghs in the fight. 1 Negh/1 Carrier being the minimum.
Had a PVP yesterday that was my Negh and 4 BoP's and the BoP's got wasted. Fast enough that my abilities were on constant cooldown. Course, the 2 sci ships popping me with subnuc/VM chains weren't helping any.
Never even had a Tac officer on my bridge until T5.
In several matches have been top damage AND healing.
At T5 my loadout is 2 DHC, 2 tricobalt torps (lets me fire every 30sec) in front, Turrets in rear.
Decloak after initial attention to the cruisers/carriers. Open up with CRF2 into the feds side. SNB and tractor to make sure he stays where I want him. At under 1km drop off a tri-cobalt torp for 34k... Makes a nice explosion.