Another thing most poeple seem to forget is that Gene Rodenberry's vision for the Galaxy class, as per the original writer's guide was that VERY few ships out there should outclass the Enterprise. The reason given is a good one. The Federation is composed of many member worlds providing research for economy and defense. The Galaxy class was supposed to be very powerful. That's probably psrt of the reason the first season was comprised of a God-like being, weak Ferengi, and alien parasites trying to outwit Starfleet. It's also the reason the Borg were introduced in the second season, as a way for Q to show the Feds how there were other, mightier races in the Galaxy. If you analyze the whole cannon you will see that overall Starfleet ships are very powerful and highly regarded, perhaps ultimately only surpassed by their crews. Keep in ming however that most references point that during the TNG time period Starfleet is actually concentrating in smaller vessels, with the explorer type ships filling a multi-role function. The exception to this would be the DOminion war when lots of these ships were rushed unfinished to the frontline.
It seems overall the Galaxy and the Rom Warbird were equally powerful in the series.
The nebula class seems to be powerful enough although clearly inferior to the Galaxy. However it was powerfull enough to survive a Cardie attack with shields down and still win the battle, as per the case of the U.S.S. Phoenix.
And about the Borg. Clearly the Federation has advanced against Borg technology since their first encounter. Remember, during the first Borg invasion Starfleet ships were impotent against the Borg. By the time of First Contact by the time the Enterprise got there the cube has sustained heavy damage. Actually, the Enterprise had more problem in close combat againt the Borg than in ship tp ship contact.