My mate lent me a crapload of Dimmu Borgir, any advice on what their best songs are if you please!

Yeah, you should ask in there. But hell, why not? For me:

For All Tid
For All Tid
Over Bleknede Blåner til Dommedag

Stormblast (original)
Alt Lys Er Svunnet Hen

Enthroned Darkness Triumphant
Mourning Palace
Spellbound (By the Devil)

Spiritual Black Dimensions
The Insight and the Catharsis

Puritanical Euphoric Misanthropia
Kings of the Carnival Creation

Death Cult Armageddon
Progenies of the Great Apocalypse
Cataclysm Children

Stormblast MMV
Sorgens Kammer - Del II

In Sorte Diaboli
The Serpentine Offering
The Sacrilegious Scorn

Those are my recommendations.
To add to that list I would say In Death's Embrace, Reptile, A Succubus In Rapture, Tormentor of Christian Souls, The Chosen Legacy, The Heretic Hammer, The Foreshadowing Furnace, The Conspiracy Unfolds. And for the hell of it, Satan My Master.
Any song that Vortex sings on.
Matthew 5:5 - "Blessed are the meek: for they shall Beherit the earth."
If you're into keyboards. Nothing beats Dreamside Dominions, I've had a boner for that song for like 3 days now.