Anything that strike my fancy is worth going into my collection.
That's the reason number one I bought Franken Fran, even though it's neither translated in French or English.
for manga worth physically owning:
Berserk (Dark fantasy)
Blame! (Cyberpunk SciFi)
Oyasumi Punpun (Slice of Life)
That's for what I would consider DISGUSTINGLY HERETICAL to read in E-book format.
To me, everything is worth physically owning, E-book shouldn't exist in my opinion.
One thing I add as a bonus:
Reversible Man (Horror/Action/Ultraviolence)
It's awesomely gory, it's hilariously violent, it's gloriously disgusting. It's only three volumes, but for that serie I am not sure it went in the American market.