From a former Mythic raider (I quit right before ToS) who's raided since TBC:
God tier
Firelands -- H-Rag still one of my favorite encounters, Alysrazor is my all-time favorite
Throne of Thunder -- Great instance all around. H-Lei Shen being one of the best designed end bosses of any instance
Original Karazhan -- Drunk Karazhan cannot be topped
Good
Ulduar -- DAE ULDUAR????
ICC -- Consistently challenging and fun encounters
BT -- Very well designed instance
HFC -- Mostly fun encounters; Mythic Gorefiend was an extremely well-designed encounter
Malygos -- Really fun encounter
Meh
BRF -- Blackhand was fun, the rest of the instance was shit
ToGC -- Filler raid, but at least the encounters were fun
EN -- We heard you liked the color red
BWD -- Tier 11 sucked
DS -- I sexually identify as Deathwing (and Ultraxion is my favorite patchwerk of all time)
SoO -- Generally acceptable instance. H-Garrosh was a pretty fun encounter.
Fuck this shit
Nighthold -- Garbage, uninspired instance with shitty encounters and some of the most annoying trash I can remember
Highmaul -- Why was this a tier?
HoF -- Fuck Vizier, fuck Ambershaper, fuck Garalon, fuck Wind Seeker, fuck this dumb fucking instance
ToES -- 3 separate raid instances in Tier 14 was just Tsulong
Dumpster fire
Trial of Valor -- Can we collectively jettison into deep space the developers who thought this abortion of an instance was worth releasing?
Tomb of Soakgeras -- Quit before it was released and I haven't heard a single positive thing about the raid from friends still playing
Anything from Vanilla -- Your raids sucked. Even Naxx. Deal with it.
MSV -- Thank you for designing Will of the Emperor, the only encounter to effectively counter weapons-grade autistic raiders
I'm probably missing a bunch. That's it off the dome, though.