It may seem cliche and uncreative, but my "forever bike" would probably be the Honda Pacific Coast 800 I own right now. It's a jack of all on-road trades, from commuting to touring to even the occasional track day. Honda designed it to be super low maintenance, with shaft drive, hydraulic lifters, and an automatic cam chain tensioner, so the thing will probably run forever with just basic maintenance.
Finally, despite numerous attempts I've made to sell it, I just can't seem to get rid of it. It's a niche bike and doesn't seem to appeal to the general motorcycling public. I'm always open to offers as I try to swap it out for a dual sport, but at this point, I'm pretty much resigned to the fact that I'll be riding my PC800 until the end of time whether I want to or not.