Behemoth was definitely the largest, with the biggest gun. If you consider raw punch, then yes,it would be the most powerful. However, the Behemoth was just one huge gun. I flew through and around her, and I didn't see anything which even remotely resembled a hangar deck, so she doesn't seem to carry her own fighters. With this, the Behemoth will most certainly need escort almost all the time when she's in hostile space. Were her shield generators and keel mounts for turreted weapons completed, she would be very tough indeed, but not invincible. A well coordinated strike can bring her down, and the Kilrathi proved just that. Even with complete defenses, a similar approach with more ordinance can accomplish the same thing.
No, I think the way to determine the most powerful isn't simply in the strength of certain abilities like biggest gun, or number of fighters, but an overall balance and intelligent use of these resources to make the ship able to survive the harshest conditions.
Even the Concordia class can be overwhelmed, but she certainly ranks high on the list. But like Azrael I'd go with a toss-up between the Vesuvius class, and the Midway class. That is of course, until Terran R&D fully figure out the Nephilim ship killer.
If they can work out the kinks, and allow a Midway class ship to keep it, then I'd go with Midway, as long as I've got first class pilots aboard.