I would recommend the following collections to a newcomer (chosen for their quality, range and accessibility):
Kingdom: The Promised Land
Button Man: The Killing Game
Leviathan
Nikolai Dante: The Romanov Dynasty
The Ballad of Halo Jones
B.A.D. Comapny: Goodbye, Krool World
Glimmer Rats
Atavar
Caballistics Inc: Going Underground
Storming Heaven: The Frazer Irving Collection (specifically for the title story and From Grace)
Necronauts
Slaine: The Horned God
Strontium Dog: The Kreeler Conspiracy
For Dredd stuff, I'd recommend a newcomer to start with one modern graphic novel (The Chief Judges Man, Mandroid and The Art of Kenny Who? are all good introductions) and one of the Case Files (numbers 3,4 or 5). This way they can decide whether they want to concentrate on the new or older stuff.