Of any band/artist.

Opeth - Bleak
SYL - Skeksis
The Devinman - Regulator
Opeth - Deliverance
Gojira - Ocean Planet

I'd add PT - Anesthetize in thar somewhere but... I couldn't!

In no order

Opeth - The Lotus Eater
Dev - Bastard
Veil of Maya - We Bow In It's Aura
Porcupine Tree - Anesthetize

This last one is a toss up between
Between The Buried and Me - Colors
or
Meshuggah - Catch 33

because they are literally both one big song.

Devin Townsend - Nobody's Here
Devin Townsend - Bastard
SYL - Skeksis
Opeth - Burden
Nobuo Uematsu - One Winged Angel

Wasn't so easy

This is pretty much impossible for me, so here are my favourite songs at this moment :

Lee Ranaldo - Notebook
Sonic Youth - Mote
Venetian Snares - Frictional Nevada
Mr Bungle - None Of Them Knew They Were Robots
Sleepytime Gorrila Museum - The Donkey-Headed Adversary of Humanity Opens the Discussion

[swervedriver]

My undisputed number 1 is Rush - The Pass.

Then the list is a bit more vague, but looking something like this:
Devin Townsend - Life
Michael Hedges - Aerial Boundaries
Swervedriver - Last Train To Satansville
Ayreon - Day Three: Pain

I tried to pick only songs that I've known for years.
In no particular order:

Moonsorrow - Jotunheim
20 minutes of pagan metal bliss. First time I heard this song was a cold winternight in a cottage lit by candles.
http://www.youtube.com/watch?v=n6f15mETiw0

Massive Attack - Teardrop
Most relaxing song there is. Even gods most be jealous of Elizabeth Fraser's voice.
http://www.youtube.com/watch?v=yftOy8kz7aE

Devin - Life Is All Dynamics
Someone on this forum said it's like "staring at the face of god". Agreed.

Jimi Hendrix - Voodoo Child (Slight Return)
This guy had balls so big it must've been a burden in everyday life.

Tanoshii Moomin Ikka - Moomin Tani Fuyu
This is the furthest back in time that I can go. Sad but beautiful emotions in wintery landscape. Brilliant. Loved it when I was 3 and love it now when I'm 19.
http://www.youtube.com/watch?v=_NU_0OH9xzo

Devin Townsend - Canada
The Devin Townsend Band - Storm
Devin Townsend Project - Coast
Meshuggah - Spasm
Opeth - Reverie / Harlequin Forest
SYL - Almost Again
Symphony X - Paradise Lost
Wintersun - Beautiful Death

I can't really narrow it down much more than that. I'm not even sure on those, but I tried to put one song by each band, otherwise i'd most likely just have a huge list of Devin songs... to be all Ass Kissy and stuff, lol.
Not sure on the Opeth one either. I love the band as a whole, but when it comes to chosing songs, very few really jump out at me when I scroll through the tracks... I always just listen to bands "album at a time" and then have favourite albums over others...
That'll have to do though

Breed 77 - Resurrection/The Battle of Hatin
The Butterfly Effect - Crave
Skindred - Kiss and Make Up
esOterica - Miranda and the Tempest
Illuminatus - The Rising Tide

Tough, very tough. I have 5 clear cut all time favourite bands. I'll choose my all time favourite from each:

Rush: The Spirit of Radio
Dev: Life
Dream Theater: Trial of Tears
Opeth: The Grand Conjuration
Porcupine Tree: Glass Arm Shattering

There has been lots of Hmming and Hahing during the think of this list. No particular order.

Devin Townsend: Hyperdrive.
SYL: Almost Again.
J.S. Bach: Tocatta and Fugue.
Led Zeppelin: Achille's Last Stand.
Muse: Hyper Music.

How ever, I'm fickle, and this list will probably change at some point.

Putting one for each band/artist, here are SOME of my absolute favourites. I can't say they are the absolute best 5 however, but it just doesn't get much better from this:

Porcupine Tree - A Smart Kid
Opeth - When
Mostly Autumn - The Gap is Too Wide
Tori Amos - Mr Zebra
Devin Townsend - Deadhead (maybe, so many choices)

Fuck! I have to have Wintersun -Beautiful Death somewhere! (Thanks to Aden for reminding...)

Tough, very tough. I have 5 clear cut all time favourite bands. I'll choose my all time favourite from each:

Dev: Life
Dream Theater: Trial of Tears
Opeth: The Grand Conjuration
Porcupine Tree: Glass Arm Shattering

Awesome!

Fuck! I have to have Wintersun -Beautiful Death somewhere! (Thanks to Aden for reminding...)

S'an awesome song innit man.

Tough, very tough. I have 5 clear cut all time favourite bands. I'll choose my all time favourite from each:

Dev: Life
Dream Theater: Trial of Tears
Opeth: The Grand Conjuration
Porcupine Tree: Glass Arm Shattering

Awesome!

Thanks!

But what do you have against Rush?

Nothing against them, I was just pointing out those songs because those are the ones I love