There's just so many choices, but I've narrowed it down to: Kiefer Sutherland, Patrick Stewart, Ron Perlman, Kelsey Grammar or Steve Merchant.
Hell, why not all of them? Kiefer and Ron could do voice over during tense moments, Patrick and Kelsey during thoughtful moments and Steve when I'm making toast/ hoovering/ all the other mundane stuff.
"Alright mate, so Ross here was making himself some toast when the postie arrived. Problem is, the bloody postie seems to have it in for the letterbox and nearly breaks it every time he rams the mail through it. Ross began to ponder how he could deal with the postie and- oh, hey Kiefer, you just gonna take over for now? Cool, I'm off for a cuppa."
Although if I could only choose one it'd probably be Kelsey Grammar. His wide vocabulary would make it interesting at least.