Updated: Sep 15, 2019
If you were to walk into my shop and ask me to teach you everything I knew, I'd have you sharpen every edge in the shop, for weeks. Not to haze you, but because the answer to the question in the title of this post is not a plane or a saw or any other thing. It's starting sharp and staying that way.
The best way I know to describe the importance of this is musically. I've noticed many woodworkers or people aspiring to become woodworkers also play an instrument- Usually a stringed one, and I'm no different. So imagine knowing all the chords, scales, rhythms, and styles, but not knowing how to tune your guitar. Imagine being reliant on someone else to tune your instrument. Imagine playing your very best chops with all the speed and dexterity gained over however many years you've been practicing with perfect placement and technique, with your guitar completely out of tune. Imagine how that would sound. You can do everything else right, but if you are out of tune, it's the only thing that matters. The same is true with your tools. They must be sharp, and you must know how to tune them.
I've written a post that is published below that contains probably way too much information on sharpening, but is worth a read if sharpening and the methods surrounding it are a mystery to you. I will say that I do believe that sharpening has become more of an industry than it probably needs to be. I've also put in the work with every conceivable method, and I've finally settled on my own which I will happily share with you. But it is not the "only" method. What is important is that you determine what is important for you, and find what works for you.
If I had it to do all over again, I would have these and only these tools for sharpening, and other shaping and maintenance operations, in order or importance:
A clamp on guide
Diamond stones in Coarse, Medium, and Extra (or extra extra) fine
10,000 grit water stone
A slow speed grinder (not the water cooled type) and an upgraded tool rest.
I'm happy to list the products that I use, and at this time I am not receiving any sponsorship from these companies. I have created marketing materials for a retailer of *some of these products, but paid for them out of my own pocket.
*Veritas MKII Honing guide
DMT Dia Sharp Diamond Stones
*Rikon Slow Speed Grinder
*Veritas Grinder Tool Rest
For the most part, I'm sharpening on the diamond plates and a strop. For the longest time, I held to the notion that I should be able to sharpen free hand, like the "true craftsmen" and I did. This is hog mess. After a while, and trust me on this, sharpening loses it's luster as the meditative experience you see in the well-lit, 4k video in a quiet shop on a beautiful solid maple bench with the quiet strokes of the steel across a water stone and a mirror finish on a blade and someone calmly saying "there" like some great achievement has been made. After a while it's a pain in the neck and you want it done as quickly as possible, or you're pulling an iron out of another plane that is still "pretty" sharp because you don't want to stop and do it. You want it done as quickly as possible, and as accurately as possible. This is where the honing guide comes in.
I use the Veritas guide, but there are several out there. What you want is some way to accurately repeat an angle. Once you have this set up, you can get back to the level of sharpness that you want in 2-3 minutes. This is what sharpening is about. Nothing else. I don't care about what grit levels and water stones and all that. And you won't either, when you're *ahem* actually building furniture, and not polishing steel all day.
That being said, I have one, and only one water stone. It is a 10,000 grit. I use it on my smoothing plane iron when I need to be taking very fine shavings, and I use it to hone my straight razor. What follows is my opinion on water stones. I don't have time to flatten them, and they waller out quick. Diamond stones don't. Water stones put a higher polish on than diamond stones. Neat. So I use the highest grit water stone that I can get, and it's easy to keep flat because I'm not using it that much. I can get that water stone polish with out all the water stone crap.
I hear a lot of great things about shapton stones. I can't afford them and I'm perfectly happy with what I have.
Sandpaper and the scary sharp method- it works. Sandpaper wears out really quick and over time, is the most expensive method out there. It's the best cheap entry to getting reliably sharp, especially with a guide, and I did it for a while. But for the most part, I'm using sandpaper and a lapping stone to flatten things, and to remove rust.
Ok water cooled grinder don't think I forgot about you. I dropped the coin on one of these and I've had it for a long while now. You can set a bevel and get a great consistent hollow grind and not cook your tools with one of these. You can also do this with a slow speed grinder, some patience, and a jug of water. I hate to tell you this, but the only way you're going to learn not to take the temper out of a tool is by taking the temper out of a tool. A set of cheap chisels from the home center and a good slow speed grinder, and a few blued cheap chisels is a lot cheaper than the original water cooled thing and their jigs. Don't guess they'll be sponsoring me any time soon. Oh well. But here's the thing, those are honing machines. If I'm going to the grinder, I'm wanting to remove some steel relatively quickly. These aren't quick. I could see it being pretty great for lathe gouges. But I'm no turner.
Novaculite or oil stones- I've used them. They work. I like the predictability and variety of grits with diamond stones better. I also like that I don't use oil on them.
All of this to say, I've wasted a lot of time and money and settled on a practical, affordable, long lasting, and efficient method that works for me. There is plenty of info out there on how to sharpen. What's important is that you do it. But don't buy into the hype and marketing stuff, and for the love of all that's wood, don't believe that you are any less a craftsman for using a guide! Look around a little bit at the builders you admire - with a few exceptions, they're using jigs, I promise. Ask yourself sometimes - is this guy building things? or is he sharpening things? Sharpening is not hard, but it is essential. The products I've listed here will get you as sharp as you need to be to do fine work.
I'll leave you with my favorite sharpening quote from my favorite woodcarver. when asked if his gouge was sharp enough to shave the hair off his arm. "I'm not carving hair, am I? I'm carving wood." -Chris Pye