The Defiant Class was the first "Warship" designed by Starfleet. Though they defined her role as "Escort" Since warships went against the "Peaceful" exploration doctrine of the federation.
Galaxy Class, Designed to be a Long Range Exploration/Science platform with a Large Crew compliment complete with modular labs. 340 degree phaser Coverage Dorsal 320 degree ventral. In other words plush crew comfort for extended missions.
Length: 642.51 meters Width: 463.73 meters Height: 195.26 meters Weight: 4,500,000 metric tons 42 Decks
Cruising Speed Warp 6
Max Speed Warp 9.6 for 3 hours..
Intrepid Class, Light Exploration/Scouting Basically the same as above Just scaled down. And intended to work either closer into federation space or as a support vessel in a larger fleet.
Length: 352.5 meters Width: 144.84 meters Height: 55.42 meters Weight: 700,000 metric tons 15 Decks.
Cruising Speed Warp 7.5
Maximum Speed 9.975 for 13 hours
So power wise, Galaxy Class but it's not really a fair comparison to be making.. It's like Comparing a Frigate to a Ship of the Line.. They each fill there designed roll better then the other could ever fill the others' roll so In my opinion it's a Tie between them..