all time favourite would probably be MAYH but if i had to pick only one to listen to forever then it would probably be either mar de grises - the tatterdemalion express or cult of luna - somewhere along the highway.
I hated this album when I first heard it because I was expecting Aenima 2, but after a while it grew on me and was in my truck cd player for three years straight. I only ejected it because it was a lease, and I got a new truck.
like everyone around here said, this is probably a question with no answer...
I have many candidates starting from Opeth, through Pink Floyd, all the way up to Alcest. Having that said, the most influential album for me would always be Opeth's MAYH, the first album Ive ever cared buying.