For example, Pentel leads run a bit harder than other Japanese brands. This makes them a bit more brittle. For a 0.3 mm pencil without a sliding sleeve to support the extended lead, I would choose a Pilot or Mitsubishi lead to reduce breakage.
Sometimes you want to make really bold and dark lines, so you go for a 0.7 or 0.9 mm with the darkest lead you can find. Those sizes are only made as soft as 2B, but the Pilot Neox Graphite 0.7 & 0.9 mm 2B is actually more like a 4B. It's the darkest lead currently on the market in those sizes. (I have some long-discontinued 6B that I use sparingly.)
When I'm writing on index cards, I usually want a harder than usual lead, writing with a sharp edge to make clean lines that won't smudge. I like Faber-Castell HB in 0.9 mm for this because their harder leads don't feel as gritty as other brands.
I have a variety of other graphite leads that I use for specific situations.
Getting into color, for its variety of hues, the Mitsubishi Uni Nano Dia Color in 0.7 mm is usually my first choice. It makes bolder lines than the same brand in 0.5 mm, because the greater thickness allows a lower percentage of structural polymer in the mix.
A big problem with the Nano Dia, though, is that it's not lightfast. Most of the colors will fade away after a month on the bulletin board. The most lightfast colored lead is the Staedtler 0.5 mm in red/green/blue. It is extremely brittle, though, so you need to use it with a sliding-sleeve pencil like the Pentel Orenz. (https://www.jetpens.com/Pentel-Orenz-Mechanical-Pencil-0.5-m...)
Aside from situation, people have different preferences in how the tip feels on the paper. Pentel feels rather slick, Pilot has a more chalky feel, and Mitsubishi to me feels rather gritty. I like a chalky feel when writing on really smooth paper and doing math, but the slicker Pentel suits me better when writing longhand on cheap office paper.
I'll bet you have favourite pencil/s as well?
The all-round best line of wood-cased pencils is the Mitsubishi Hi-Uni. The Tombow Mono 100 series was a consistent second, but I heard quality changed after moving production out of Japan. Some people like the Faber-Castell 9000 series because its hardest grades are smoother than other brands. The Staedtler Mars Lumograph series is known to give a particularly rich black with the softest grades. That may not be relevant to you if you just want them for writing, because I find it practically impossible to write with a wood-cased softer than 4B.
Some fanatics claim that the discontinued Sanford Blackwing 602 was the greatest pencil ever. The lead hardness was about 4B and was unusual for including wax with the graphite and clay. The point wore down quickly, but the lead was particularly resistant to crumbling and it was very slippery on the paper. The line on paper was not as dark as you would expect from a 4B. I would say that the best current pencils give a better line but aren't as slippery. There have been a couple attempts at reproducing the Blackwing, but I've heard mixed reports of how successful they were.
My philosophy of buying pencils is that since there is so little absolute difference in cost between the best and the mediocre, there is no sense in trying to economize there.
