The Perfect Locomotive

Another simple topic, which engine do you feel is the closest to perfection in terms of speed, horsepower, reliance, etc.?

I would say the EMD F40PH.

Ain't no such thing.
Can't be. Locomotives do different things, which require different characteristics. Approach perfection for one application and you get further away from it for another.
Simple example (assuming you want American diesels). Suppose you want to haul heavy coal trains over a route with some hills and some plains. A big, six-axle, AC motored, unit is the closest to perfection you are going to get. (As a GE fan, I'm going to say an ET44, but that's beside the point.). Suppose, on the other hand, you want to run a short line on a route with lots of sharp curves, and only a few dozen cars a week in traffic. The big CC unit will be hell on your track, will derail on too many curves, and anyway is way more powerful than you need. For YOUR purposes, something like, say, an ex-CR, ex-PC, RS3M might be closer to perfection.
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Things are even clearer with steam. Suppose you want to run fast passenger limiteds between New York and Chicago. If you're railroad has a water-level route, a Pennsylvania K4s would be a lousy choice (oversize cylinders that demand more steam than the valves can handle at high speeds, for a start), but a New York Central J3 might be close to perfection. If, on the other hand, your route goes over the Alleghenies, a J3 would be a flop (boosters are fine for starting up, but the J3 doesn't have enough tractive effort for sustained hill-climbing), but the K4s... well, maybe not perfect, but gave decades of good service. Which locomotive is closer to perfection? That all depends on what kind of perfection you want!

I'm not limited to American diesels. You make good points about perfection, and how each locomotive would be good for a different purpose.

With that in mind, what locomotive would you say has the most purposes, and performs best in most conditions?

For a locomotive which performs best in most conditions, I'd choose a GP38-2. They have been used in just about every type of service, and they are very dependable.

The GP38-2 isnt too far removed from the F40PH, which is more or less a GP40 with a carbody. Main difference seems to be the 40's having a turbo, which is an expensive nice to have for freight units.

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Actually any 40 or -2 EMD gets my vote. I mentioned the GP38 because of it's lower horsepower being more suitable for switching and local service. A GP40 would do just as well, but the extra top end horsepower was a bit overkill for these types of jobs. Why not include the 39? it fits right in between the other two models.

One other good model was the early GP series. They were used successfully for about everything. They lasted in class one service until recently, and maybe CN still uses some. I tended to like running them.

Although not EMD, the Alco RS2/3 were a good all around locomotive. They didn't have the support that GM had set up, and also had teething trouble with the 244 diesel. My employer used them on everything from yard jobs, to the fanciest limiteds.