Similarly to George, I too have only trained in Japan briefly and base my answer on the experiences of others who have trained in Japan extensively. What I mean by that is for longer than two years.
I think the key to "best training" lies within the trainee. If you're dedicated enough to travel to a foreign country, be treated as a gai-jin, learn the language and deal with all the other rubbish in order to train with people you regard as luminaries of budo... you're going to be training at your pretty darned best and make every moment of training count.
The people with that kind of dedication take with them the "best training" experience that they discovered in Japan when they go home. Occasionally I glimpse it and it's most inspiring.
If you're travelling to Japan as an aiki-tourist with the perception that training's going to be any different from what you've had at home, I think you're going to be sorely disappointed. The location should't make that much difference.
I once met someone (with 20+ years experience and practice in several countries) who trained a few times over a period of months with Chiba Sensei and took very little out of it. As talented as I'm sure Chiba is, he seemed pretty shallow to this person. A pretty sad story all around if you ask me.
The best training for me - particularly at a foreign dojo - is where I come out stimulated with more ideas, and made more connections with people. Thinking about what you do, and getting that feeling that you've really made contact with someone at a deep level, just makes me buzz when I come back home.
Liam - Uni of West Australia Aikido
ps: I think Jun's polls are great but not because of any factual accuracy they convey. For at least a fraction of an instant it challenges me to think about something aikido-related. Then it's that time between pressing the answer button and waiting for the response that I enjoy the most, as it lets me wonder about whether (any!) other people also feel the same way that I do. One person is enough!
