I'd have to say that the "best" engine I've owned has been the 1ZZ-FE in my current Corolla. It's very quiet, smooth, efficient, and powerful for its footprint. From everything I've read about these, any example ought to be good for at least a quarter million miles, requiring just a little bit of gas and oil to get there. I've lived "in excess" before, but I really appreciate how frugal the 1ZZ-FE is, while sacrificing little to get there.
But my "favorite" engine is the Cadillac Northstar. I've owned two; one with 160,000 miles when I sold it, and one with 70,000 miles when I sold it. The wail of that engine screaming to 6,500 RPM was rather habit-forming. And despite fears of working on them, it was a right easy engine to maintain. A starter change (never had to do one) is an easy job under the dry intake manifold (no laying on your back under the vehicle). A water pump change (did have to change one) is very easy as well, about 30-45 minutes. And the durability of these engines is excellent as well. Some did have some head bolt issues, but most are pretty solid.
I think GM really missed an opportunity to keep that engine current. There were really two generations; the 93-99 and the 00-current (for FWD anyway). But the power didn't really change throughout its run. The LD8 made 275/300 for hp/tq, and the L37 version made 300/295. And that's pretty much the way it was, and still is. That engine was a real ground-breaker when it debuted, but now it's lagged behind most every other premium auto maker, and I believe GM plans to let it die a slow death. I wish GM turned that Northstar into a 4.8-5.0 liter Q-ship that stood for everything that was good about GM and Cadillac.