Telling the difference:

Assuming you are talking 2 cycle LB engines, If it has 3 mounting holes and sits flat on the muffler plate it is a "C" or "D". If it does and the recoil pulls sideways, it is a "C", IF the recoil pulls straight up with 2 fingers it is a "D" then if it has points it is a D400 if it has CDI it is a D600. IF when you look at the carb side the engine looks like it is standing up off the muffler plate on 2 spraddled legs like a Sumo Wrestler it is an "F" or a "V" then if the engine cooling fins are round like the cylinder it is an "F", if the cylinder cooling fins form a square it is a "V". Muffler out the side an "E" or DuraForce.

you may want to add in the "D" part, if there is just a single re-enforcing web in the transition between the cooling fins of the cylinder and the cooling fins of the head, it is a "C" or "D400". IF the web forms a capital H, it is a D600, "H" for high compression. While it will interchange with the cylinder jugs of D400 and late "C"s, do not do it as the flywheel cooling fins can not handle it. I found out the hard way. On the other hand, a D400 cylinder jug will go on a D600 OK, just loose some power.

An "M" doesn't resemble anything else.

Also, early "F" series had the "two finger" pull up start but do not fit the other qualifiers.

I am not going to start to tell the differences other than D600 and up, "F", "M", "E" all had CDI (unfortunately)

The 'V' was the first 2-cycle design after Toro took over. As Walt said, it has squared cooling fins, but the quick ID is it has a one-piece cylinder - no removable head. The 'E' engine (Duraforce) was the last 2-cycle made by LB. It looks like the 'V' except it has a four-bolt removable cylinder head. Just a minor point - Walt said the Duraforce muffler is 'out the side' - it actually is 'on the side' but the exhaust still goes straight down out of the cylinder to under the deck, and comes back up into the side-mounted muffler.
The early engines had babbitt bearings instead of roller bearings, and I wouldn't consider those in contention to be 'the best'. The 'F' engine was probably the most-produced and arguably the best. The 'V' was a cheaper-to-produce version with a few problems. IMO the Duraforce was the ultimate LB 2-cycle mower engine. All of these engines had iron cylinder liners and after some early point they all had roller or ball bearings. This is why all the modern LB engines were commercial-rated. The commercial mowers had the exact same engines as the residential because they were already 'commercial'. So, out of all this I would say the 'F' or 'Duraforce' would be the best. BUT....the oil-injected 'M' was the technological tour-de-force, and mowers with the 'M' engine have never been equaled. Technically, this engine was the best, but nagging problems and failures in the oil injection keep it out of the running for 'the best'. Lawn-Boy was truly the innovator. Not surprising since Lawn-Boy was the first commercially-viable rotary mower. They just kept coming up with great new ideas. They invented what Toro now calls 'Personal Pace' for example. Actually, the mower world is very boring without Lawn-Boy innovations and the 2-cycle outboard engine adapted to a mower.