I used to live in (the smaller) of two Portlands in Australia. The locals still tell of the hapless truck driver who pulled over to ask street directions, and was told he should be a thousand miles away. 
Then again, as a postal worker, I have lots of these horror stories. For a country with such a small population (so fewer towns), I still can’t work out how we wound up with SEVEN Mt Pleasants.
These days, Maine. I have a really good friend who lives there.
Tennessee…
No, actually my first thought goes to Maine, because I was in college a little north of Portland. Then it goes to Tennessee. Most of the time I don’t even consider the one in Oregon to be an option.
If you just said to me, "Portland, " and expected me to do free association, then Maine, since my family vacations a little over an hour North of there every summer. It’s a very pleasant town, although living in New York I have trouble calling it a city.
If I saw something on the news, or someone mentioned something happening in Portland, I’d assume Oregon, since obviously most people assume Oregon when they hear Portland, and that’d probably be what somebody meant when referring it statelessly.
It would be Oregon for me, and for a majority, I suspect. But in parts of the U.S. I’m sure there are quite a few people who don’t roll to the majority default when they think of Houston, Paris or Las Vegas.
Maine. I go there, the radio stations I listen to are located there… I’m always disappointed that people on the SD mean Oregon when they mention Portland.
Maine, definately.
[Bahstin Accent]Does this road go to Portland? [/BA]
[Maine Accent]Nope, stays right here.[/MA]
Oregon, but my father is from Eugene. I associate Portland with very frail, sweet old ladies, since as a kid we’d fly into Portland to visit my great-grandmother.
I hear they have some young folk now.
And by the way, Interrobang!? is a really great username.
Cement.
Don’t get me started on f***ing Powell’s. (I’ve got a rant brewing in the back of my head—coming someday to a BBQ Pit near you!)
Anyway, since I live there, Oregon for me.
Powells. Home of the most arrogant snitty rude unhelpful my-god-I-am–much-too-good-for-this-pannyanny-retail-job employees anywhere in the universe. I’ll join yer rant, Ferrous old buddy old pal. Any time.
Portland, Oregon, being my home now, is my default Portland. Oddly, I grew up on the east coast and never heard of the one in ME until I moved to Oregon.
jackelope I think I haven’t seen anyone over 50 since moving here. I swear to the sky this has to be the youngest-average-age city I’ve ever seen (I live right in the city, though.)
Oregon. Mostly because I live in Tigard.
Portland, Dorset (UK) is the only true Portland, naturally. 
Even though I was born in Maine, the only appreciable amounts of time I’ve ever spent in that Portland have been getting to and leaving the airport (in relation to family reunions, which are always spent near Moosehead Lake when we do have reunions in Maine.)
So, to answer the OP, I’ll have to say Oregon, mainly because I have relatives who live near it.
Whereas there are no relatives living in Maine; the extended family is originally from there (thus the occasional reunions), but everyone moved away many years ago.
Oregon.
Not surprising, as the first place that i lived in North America was Vancouver, B.C., and i’ve travelled quite a bit in the beautiful Pacific Northwest.
I think Portland Trailblazers first.
I live in PA. I always think Oregon.
Oregon. Mostly because of “Night of the Living Rednecks” on the Dead Kennedy’s Give Me Convenience or Give Me Death. And Jim Goad.
Though I’ve learned to be careful about it. In junior high, I wrote a paper on the murder of an Ethiopian immigrant by some Portland, Maine boneheads, but referred to Oregon the whole time. Luckily, my teacher wasn’t the wiser.
I live in Cincinnati.
Maine, because I grew up there (in the state, not in Portland).
Oregon. It’s the only one I’ve been to, and the one that I’d heard of before I moved to New England.