As a Christian myself, I listen to quite a lot. You're right about a lot of bands being pretty bad, but here are some that I'd recommend (a lot of these have been mentioned already):
-Theocracy (prog-power)
-Divinefire (power with some growls here and there)
-Anything else that Divinefire singer Christian Rivel/Liljegren has ever done, though Divinefire is the best
-Tourniquet (prog-thrash; early stuff is closer to straight thrash, later stuff has more classical influence)
-Believer (prog-thrash)
-Extol (prog-death, though every album is different; Burial has some black elements, Undeceived has a bit less, Synergy is essentially tech-thrash and The Blueprint Dives I've heard described as Porcupine Tree, Muse and Cult of Luna put together)
-Antestor (black; early stuff is death-doomy, The Forsaken is melodic black and the new one sounds kinda boring to me)
-A Hill to Die Upon (blackened death with the best growls I've ever heard)
-Lengsel (progressive/experimental black)
-Becoming the Archetype (progressive death/metalcore)
-As I Lay Dying (I know, no metalcore, but An Ocean Between Us is really good)
-Stryper (glam/traditional, obviously not to everyone's taste)
-Vengeance Rising (thrash; I've heard them described as the Christian Slayer)
-Crimson Moonlight (intense melodic black/death)
-Paramaecium (death-doom)
-Immortal Souls (melodic death)
-Harmony (prog-power)
-Deliverance (thrash; went a bit proggy later on)
-Impellitteri (speed/shred)
-Rob Rock (Impellitteri singer's solo project; traditional/power)
-Pantokrator (death)
-Mortification (death; don't go for anything after Post Momentary Affliction)
-Renascent (melodic blackened death)
-Cage (the continuation of Painkiller)
-Veni Domine (epic doom)
-Seventh Angel (thrash/doom; believe it or not, this combination exists)
-Sacrificium (death)
-Benea Reach (progressive metalcore)
-Virgin Black (gothic doom)
Stuff that's hard to find/bands that have barely released anything:
-Schaliach (melodic death/doom duo)
-Vardøger (black)
-Wintersoul (Crimson Moonlight turned up to eleven)
-Monotheist (progressive death)
-Mantric (weird project in the vein of Extol's last album)
-Grave Declaration (melodic black)
-Majestic Vanguard (flower metal)
-Evroklidon (black)
And who could forget that most famous of Christian thrash bands, Testament?
Well, that's about all I can think of. Some well known ones and some underground ones. Pretty much everything on there is YT-able, but a lot would be very difficult (if not impossible) to get hold of on CD. If you're still after more and don't know where to look,
the annals of my Last.fm library might be a good place to start.