Top 5 Five mans..

[Omertocracy]

In no particular order since I cannot decide between them.
Halls of Reflection
Scholomance (old)
BRD
Zul'Farrak (first thing I ever saw of WoW was my brother's first run through here)
Shadowfang Keep

[sheffield]

5. vanilla UD Strat for 45 min Baron runs
4. RFC for being the first
3. Old Hilsbrad Foothills - only because up to that point, I had never really played alliance, so it was cool playing as a human
2. H-Alcatraz - very challenging at the time, helping guildies get attuned, fun times were had
1. ZA, but since that was originally a 10-man, i will list the Cata version as my favorite, since it was basically the same, but less people and much easier

honorable mention goes out to SM Cath vanilla version.. dunno why, just like the layout i guess

[rytx]

1. Magisters Terrace
2. Culling of Stratholme
3. Zul'Farrak
4. Halls of Reflection
5. Halls of Lightning

[Rkd]

5)Stormstout Brewery
4)UBRS
3)Zul'Farrak
2)Halls of Reflection
1)Deadmines (vanilla)

[Drfireburns]

5. Uldaman
4. UBRS
3. Vanilla Scholo
2. Vanilla ZF
1. Tie between Shattered Halls and MGT.

[Deleted]

i definetely don't agree with black morass type instances. they usually meant a lot of waiting and little action, much like violet hold and whatnot. shattered halls was good yes, but i wouldn't call it the top. on the other hand, i would definetely put dire maul and maraudon on this list. imho there are more than 5 dungeons that are very good and very close fun wise. from old deadmines to stormstout brewery i enjoyed many instances throughout all add ons.

[MOEEEE]

1. Ragefire Chasm
2. Deadmines
3. Violet hold
4. Utgarde Keep
5. The Arcatraz

[Slant]

Blackrock Depth
Dire Maul
Lower + Upper Blackrock Spire
Stratholm Living + Undead
Scholomance

Why? Specifically, because they took more than 15 mins to clear.

[Faesroll]

Zul farrak
Blackrock depths
All the scarlet monastery wings

Nothing else really comes close to those, the ICC 5mans were quite fun as well though.

[Vantricular]

1. Halls of Reflection
2. Magister's Terrace
3. Pit of Saron
4. Botanica
5. Shadow Lab

[Renisis86]

5 - Classic - BRD - The size AND lore really brings me back to the beginning of my WOW experience.

4.- Zul Furrak- Again another classic instance. The Event always seemed so epic seeing the hordes of people show up cornering you on the steps. Plus the endless rungs to get your your first epic, even if it wasnt any good for your class.

3.- BC Heroics - I know this isn't just one but I didn't want to round off the top of my list with a bunch of bc heroics. These heroics took skill to complete. Engaged everyone in the party. Yeah most people face rolled these by getting Mages to Sheep. Train trapping mobs that would one shot you was always fun and something to be proud of. LFD wouldn't likely work well for these but, this is when the add-on Call to Arms first got big and pugging these heroics during the middle to the end of the xpansion wasn't so bad but still required a skilled group. Also Ill add this is also at a time when people had to play there best in a pug group, because if they were terrible, or had a bad attitude the whole server could find out. Especially on small server.

2.- Halls of Lightning - Had that epic feel you want in a 5man. Great aesthetics, and fun boss encounters.

1.- They Haven't made it Yet.

[melzas]

1. Halls of Reflection - my favourite 5man, hard, good lore (with different hero guiding you depending on your faction), awesome music, great instance... also loving Pit of Saron and Forge of Souls
2. End Time - again, good lore, fun boss mechanics
3. Halls of Origination - my favourite cata 5man, love the amount of bosses
4. Magisters Terrace - mostly because Im belf fanboy
5. Grim Batol - because I like dwarfs and hard dungeons (and it was one of the hardest at the start of cataclysm)

[vitalic]

*Heroic Shadow Lab- Random orc ninja attacks! Such a ball-buster, never had more fun in BC. Don't think that I ever actually beat heroic Murmur though.

*Heroic Ramparts- one of the easier BC heroics, again I loved the tough pulls around corners, having to monitor for patrols (that damned wolf master!)

*Upper BRS- Perhaps the most epic small instance ever put together. OKay okay not really 5-man I know. The pinnacle of the long, complex vanilla dungeon with an actual story and atmosphere. Plus, the only dungeon I ever experienced where I got to mind control mobs into bottomless pits. "Does anybody have the key?" /facepalm

*BRD- An entire world unto itself. Of course totally impractical; I never came close to succeeding in it when it was relevant. It should have been lvl 60 given it's depth and difficulty.

*Deadmines- another imaginative story/adventure-based dungeon. Man those vanilla dungeons were creative!

[McNeil]

1. Classic Stratholme
2. Classic Scholomance
3. Classic Scarlet Monastery
4. Culling of Stratholme
5. Halls of Reflection

Notice how much I liked the vanilla dungeons over the updated ones

[Deleted]

BRD
UBRS
LBRS
STRATHOLME
VANILLA SCHOLOMANCE

dont know something about those dungeons makes me love them, guess its due to their length, I hate fast dungeons (well not if farming valor I guess)

[Slowpoke is a Gamer]

Temple of the Jade Serpent, Well of Eternity, Mechanar, Arcatraz, Scarlet Monestary.

[Role Unknown]

Uldaman...easily tops the list, "Who dares awaken Archaedas?" I always wanted to make that noise he makes during the encounter.

Shadow Lab...first time i saw Murmur i was mesmerized....til he kicked our asses.

(Classic) Scholomance

Zul'Farrak... loved the pyramid event

The Oculus; Im probably the only one who likes it. It wasnt hard if everyone knew what they were doing.

[Vyrd]

5. shadowfang keep vanilla. the remake stayed pretty true to the original. i loved the fact that it felt like an actual castle. every room served a purpose and you had to fight your way to the top.
4. scarlet monastery vanilla-graveyard, library, cathedral and armory. like shadowfang keep, the entire place felt functional. i like what mop did with binding the two together for the sake of re imagining it as a full compound, but vanilla scarlet monastery will always be my favorite. i leveled my hunter in vanilla from 30-42 doing only SM runs with a group of friends.
3. UBRS vanilla-i loved how you could clear it with a 10 man group, it really felt like a challenge. killing rend blackhand, and then paving the way for blackwing lair (one of my favorite raids). i wish blizzard would bring back 10 man dungeons. have them be one step below a raid and one step above a heroic. it wouldn't be a huge hurdle from 5 heroics to regular raiding, but the 10 man dungeon gear would supplement the gear gap.
2. throne of the tides-visually one of the most impressive dungeons ever created. the hallways are empty, but then naga burst from the water barriers to ambush you. the final bossfight was the only part i didnt enjoy, and erunak (the draenei boss that everyone skipped) felt wasted. i really hope that blizzard expands on the neptulon story, because throne of the tides set it up so well. it was challenging and a quick clear. ill always laugh when the second boss bursts through the roof and makes that noise. http://youtu.be/SJWeOxfVQjo?t=1m24s.
1. heroic shattered halls-if you did not have a paladin tank or a mage, you did not complete this instance, at least until people could outgear it. the timed incentive of saving the prisoners, stealthed mobs that will hunt your healer down. trashpacks that you had to wait for them to nearly kill eachother before you could pull them. kargath fucking bladefist. i loved this heroic, especially because i played a paladin in TBC. the trinket from kargath allowed me to be invincible while it was up, and i could farm the demon hunter trainees near black temple for cloth, by the pulls of 30+.

honorable mentions
caverns of time-all of them
mechanar-the reason blizzard made heroics have a daily reset
magister's terrace-toc babies gfy, this was the original pvp encounter in a 5 man.
shadow labryrinth-murmur boss room when you first open the door.

[Coldhearth]

I must agree. BRD back in the day was insanely fun, dangerous, and just plain awe-inspiring.

Getting a group of friends together and exploring every hallway, room, and corridor. Took hours and hours. Mobs, hard bosses, loot, and treasure around every turn. Getting lost but having fun regardless. The place was expansive, dark, mysterious... and was the embodiment of the "dungeon crawl" from the pen and paper RPGs.

Too bad now "dungeons" are just boring, linear hallways with the gogogo attitude.

BRD and BRS were truly dungeons in the fantasy sense of the word. Wish we could see more of these types of experiences in the future although I don't think we'll ever see their return in modern day 'world of queuecraft'.

[Mehrunes Dagon]

5. Shadowpan Monastery
4. Vortex Pinnacle
3. Deadmines 2.0
2. The Stonecore
1. Halls of Reflection