Well, if "last forever" is key, you might give Titanium or Inox (stainless steel) first consideration. Sure, other steel can last as long as you maintain the paint (and galvanizing on the inside), but Ti or Inox leave you with one less thing to worry about. Ti certainly seems to be the preferred material among the folks I know that get one bike and ride it everywhere forever. Interestingly enough, although Inox is commonplace in day-to-day stuff (hardware, household goods, etc.), it's more exotic than Ti when it comes to bike frames.
Take a look at the Kona Rove Ti. I test rode one and found it mighty sweet - I might have gotten it if I hadn't just put together a steel bike - the shop owner just wanted me to try it. Just checked to see if it was still in their lineup, and found their blurb to be aimed at you:
The same freedom to ride everywhere as its steel and aluminum brethren, but in Titanium. Basically, it’s like riding a Rove on a cloud. Lighter, with better damping. You could have this bike for the rest of your life, the same luxurious ride feel as the day you bought it. That good.