Best data archival media?

I was wondering if you gurus could share some impressions regarding
the best media type for data archival.  In particular which media type
would you trust for archival of important data you wish to take
off-line >10 or 15 years.  Would you suggest tape like DLT or some
optical disk or MO?

I'm asking for impressions because, at this time, no one has 15 or 20
years experience with, say DLT IV.  While capacity, price, speed are
important to decision making, I though I might start out to ask for
just general reputations and strategies.

Thanks