I'd say the Galaxy class is one of the strongest ships seen used by Starfleet. Sure, when it's shields were down (ignored) in Generations or when it's facing a new type of weapon like in TNG's "Darmok" or DS9's "The Jem'Hadar", it's defenses will be compromised but after upgrades, it can take whatever you dish at it. If you put the Enterprise D featured in "Encounter at Farpoint" against the Odyssey or even the Enterprise D in "All good things", they'll trump it. IMO The Galaxy class is as upgradable as Voyager if not more and we've seen how different Voyager was after it returned. Honestly, if Janeway had a Galaxy class starship, I bet she would have gotten home sooner (handling transwarp or utilizing Borg and Starfleet transmission upgrades better).