Whoa, 10 points awarded for digging up an ancient thread from 4 years ago but I'll throw in my two cents.
As far as classic psychedelics go (LSD, Shrooms, Mescaline), for me there is definitely a light psychological addiction in the sense that I love them and want them around in my life generally. Kind of like saying I'm addicted to fun. There's definitely no physical aspect to this addiction because I can go without them indefinitely, but will miss them like I miss an old friend.
I would definitely be very sad if I was told that I can never again trip balls on a classic psych.
Is this addiction?
When it comes to patterns of addiction in the classical sense (need, craving, withdrawal, etc.) as far as the greater family of psychedelics go (in a broader spectrum outside phens trypts and ergots), I would definitely vote for dissociatives being the most addictive; PCP, DXM, Ketamine and Nitrous Oxide.
I have also heard that certain psychedelic phenethylamines such as 2C-I tend to lead users into patterns of repeated and/or chronic use.