Here's my list. Solely based on reliability and personal experiences.
Reliable:
- Jeep/AMC 4.0L Strait 6
I've seen so many examples of why this engine is absolutely rock solid. I have a friend who drives a Grand Cherokee with one and it's currently at 280K running without issue. My previous manager had one with 230K miles and it ran like a top. Lastly, my brothers father has a Cherokee that he drives all day long and beats the hell out of (180K miles) and you'd think it was brand new. Gas mileage isn't horrible, with about 21MPG highway.
Buick 3800 Series 3.8L V6
GM put this engine in nearly all of their vehicles in the late 80's all the way up to the mid 2000's. Lumina LTZ, Monte Carlo, Impala, Intrigue, Regal, Le Sabre, ect, ect. It's rock solid, produces anywhere from 170HP - 210 HP. I had a co-worker with a Lumina LTZ that had 245K miles without any major engine overhauls. The only issue with this engine is the fact that it's paired exclusively with automatic transmissions and nearly exclusively put in FWD vehicles. Granted, some Getrag transmissions from Fiero's mount up to this engine, it's not a stock configuration. MPG tends to be about 27 highway.
Toyota 2.4L 22R(E) L4
Arguably one of the most "well known" engines for it's reliability. The engine is found in some 80's model Celica's and nearly all 80's and 90's Toyota pickup trucks. Nearly all of the Toyota pickups with the 22R(E) are found with 250K+ miles and they are still running. I have a friend who personally has a 1991 Toyota Pickup with 290K miles on it and drives it to work and college classes daily. The only real downside is it's not-so-impressive fuel economy.
Toyota 4.0L 3F(E) Strait-6
This engine was found primarily in 1985-1992 Toyota Land Cruisers. Their slow-revving design made it extremely common for the engine to reach over 300,000 miles without issue. They where gutless (155HP) horridly slow and ate gas like it was their day job (About 12 MPG city and highway regardless) but they lasted forever. A good friend of mine purchased a Land Cruiser with 270K miles on it and the thing was as reliable as a rock.
Ford "Modular" 4.6L (2-Valve version) V8
This engine was in Mustang's, Crown Victoria's, F-150's, and E-series Vans. It produced a measly 225HP and had a horrible spark plug design, but it's dead-reliable if maintained properly. These engines are often abused by police forces for 200K+ miles, then they are sold off to Taxi companies to drive another 200K+ miles. This engine was exclusive to rear wheel drive vehicles.
Suzuki G13A 1.3L (8 Valve) L4
This engine was found in North American Suzuki Swift pre 1998 models and all North American Suzuki Samurai's. It's a light, all aluminum engine that only produces 60HP but has proven itself to be an extremely reliable engine. Not only is it reliable, but it's cheap and easy to work on. I had a friend 2 years ago with a horribly abused Suzuki Swift that ran just fine at 222K miles with no engine rebuild. Eventually he sold it to his troubled brother for $80 and it's still running around today.
Toyota 4A-F(E) 1.6L (16 Valve) L4
This engine was in a plethora of Toyota vehicles. Tercel's, Corolla's, Celica's, ect. It's not uncommon for vehicles equipped with this engine to hit 300K miles before needing any major overhauls. The fuel injected models deliver 113HP which is pretty impressive for the displacement size. They're extremely easy to work on and cheap to replace. The engine was exclusive to front wheel-drive and 4WD Toyotas.