... Longevity, simplicity, ease of maintenance/access?
Using the above criteria as the predominate points, I'd have to say ...
Ford 3.3L n/a v-6 in the F150. Hands down.
Longevity: This engine is part of the Cyclone engine series; a very robust and long-lived engine with no real issues (other than water pump, which I'll discuss later on). These engines make enough power to be very usable, but not so much as to stress themselves.
Simplicity: It's got DOHC with DAMB, no turbo to leak or have wastegate issues, it's PFI, etc.
Ease of maintenance/access: Because it's in a normal truck chassis, it's longitudanly orientation is the epitome of simple. Nothing complicated on this engine and everything's easy to get at. Further, while the transverse version of this engine has some waterpump issues, the longitudinal system is MUCH simpler to work on, and for some reason seems to have fewer pump issues.
Added bonuses: decent economy; aged enough that problems are pretty much non-existent and parts are readily available; UOAs for this engine series shows very lower wear rates historically