I think I would just die if I could only have just one. Actually all planes do pretty much the same thing, peeling off a thin shaving of wood from a board and ideally doing so without tearing out. So in theory you could get by with just one. But by changing parameters of the design, like length, width, cutting angle, a plane can be better at one kind of planing than another. I don’t think you are asking for a dissertation on how planes do different things. I think you mean which plane is worth spending extra money for. It would not be your only plane, but others might be used tools or lesser priced brands. For this, I say Lie-Nielsen No. 4 in bronze. It is a beautiful tool and it is a ready to go right out of the box. You can trust it to do that very last pass without pulling out a big chunk of wood and ruining the surface. You still have to be able to get the blade really really sharp and set the chip breaker close but the work of mating the chip breaker to the blade and the blade to the frog is done by Lie-Nielsen. Did I mention it is beautiful too? It is worth saying it twice.
If I misread your question and you really meant which is the best all around plane and you want to spend extra on the one you will use the most, then it is a No. 5. It is a great all around size. It is long enough to joint edges up to 20 inches or so. It can be fitted with a cambered blade and used as a scrub plane or with a finely sharpened straight or lightly cambered blade as a smoother. You can lay it on it side on your shooting board. Which brand? All the premiums are good. I have three No 5’s: bevel down and bevel up from Lie-Nielsen and bevel down from Veritas/Lee Valley. Of theses choices, I would pick Lie-Nielsen bevel down. I prefer the traditional Stanley adjuster. I don’t like adjustable mouth planes. It is too easy to nick the blade if the mouth is set too close. For smoothing, a bevel down plane with close set chipbreaker works better than bevel up.
I personally bought the L-N No 4 first about 20 years ago. I have added greatly to the collection since then. I expect you will not stop at one premium plane. I have said to Janicewhokeepsmehumble that my goal in life is to be able to go to a Lie-Nielsen Hand Tool Event by walking into my shop. She rolls her eyes.
I would go with a LN #8. I think the quality and results from it are harder to replicate. Not impossible, just it is nice not having to fuss with a long plane.
While there are a million ways to square a board, and it certainly isn't essential, I love my LV shooting plane. That is truly a great plane.
I suggest you visit this site before buying a plane.
https://search.yahoo.com/yhs/search?...bedrock+planes
All of them .
Slightly kidding, but I have very little time to work the wood. For me, the biggest return on investment is being able to actually get to use the tool in the time I have allotted for woodworking. I know some of the seasoned vets here can get a beater plane slinging 0.001" shavings in less than an hour, but I'm sure it took them quite a while to get that proficient at it.
If just one, though, I'd have to go jointer. But which one? It would probably be the LN #6. I do have the LV BU jointer and the LN 8, but those don't seem to come out much. Then again, the only "large" project I've worked on was my bench.
LV or LN block plane.
Michael, fitting the cap iron to the iron can take only a few minutes, there are plenty of instructional videos on how to do that. I would try that first before I went ahead and spent $400 on a new plane, plus probably another few hundred dollars in sharpening gear because now you need to sharpen harder steels.
If you don't appreciate the plane I would buy a second hand plane from Stanley or Record. Once you are able to appreciate the plane then move to Veritas or LN. Then the price does not seam so crazy.
Is it necessary to buy high end planes. No. But it can make your life easier.