This is going to be long winded, be prepared.
The thing about steel is, it varies from manufacturer to manufacturer and even knife to knife. My favorite steel is CTS-XHP, but I like PM steels in general.
If you sharpen yourself, especially freehand or with a sharpmaker, etc, the balance of edge retention and ease of sharpening is very important.
I like variety, brand, color, materials, etc. I own several knives with CPM-S35VN, all different manufacturers. Let me tell you, each one has been different in edge retention and ease of sharpening. Blade grind and geometry play into this as well.
For example of my S35VN knives:
Massdrop/WE/Ferrum Forge Gent Select. $120. Full flat grind, sharpens easily, gets very sharp. However, the edge retention is the worst of any S35VN knife I own, more on par with non PM steels IMO.
Southern Grind Spider Monkey $225 ish: Full flat grind, fairly easy to sharpen, excellent edge retention, the best I've experienced in this steel. It also gets very sharp.
Three SIsters Forge Beast (no longer own) Very obtuse flat grind, was pretty easy to touch up, average retention, somewhere between the aforementioned.
Is it because one is made in China and the other USA? Not sure. Could be one is made in a much larger factory as well. I've yet to need to sharpen my Brian Tighe and Friends Twist Tighe OTF or Heretic Manticore E in this steel, but edge retention has been similar to the SG SM. M
CPM-S30V has been all over the board for me, but once past the initial edge, my PM2 has been fantastic, as has the Yojimbo 2. Sharpened and touched up occasionally. More difficult to sharpen than S35VN, but not difficult. My Hogue Exploit I believed lied about being S30V because its incredible in edge retention.
M390 (and its derivatives like CPM-20CV and CTS-204p) is a very mixed bag, though as a whole is considered a fantastic steel. The Italians (MKM, Lionsteel, Viper, Fox, etc) are known for running this steel softer than what is ideal, but in that I only have one of these knives and rarely carry it, I cannot comment.
I've yet to sharpen any of my M390 knives, but I have been very impressed by the Spyderco Southard (CTS-204p), WE Deacon (M390), and A Purvis Progeny V2 (M390). My ZT 0470 (CPM-20CV) is too new to give any input.
However, when it comes to that "perfect" combo of ease of sharpening and edge retention, thus far CTS-XHP is my favorite.
CPM-M4 and CPM-S90V were difficult for me at first, but as I got better at sharpening, the easier it became.
I suggest starting with one of the "lower end" CPM steels, learn to sharpen it, and them progress to the "super" steels etc.
Tool, Stainless, Carbon, really depends on environment, body chemistry, and how you carry. Me, I live in Chicago, and it has to be 90 degrees plus for me to sweat a drop. I have no worries of corrosion on my knives. However, if I lived down in Galveston, TX (island in the Gulf) and had major sweat issues, tool or carbon steel would be off the list.
You will hear many opinions about how some steels will rust just looking at them, like CPM-M4, yet my Spyderco Gayle Bradley in that steel sat in a drawer for over a year without a spec of rust.
In the last few years, production knife companies have been making their products in handle materials that were previously only available via custom makers, especially micarta. I personally prefer smooth handles with good ergonomics over stuff like G-10 and FRN. Custom scales are fun especially if you want to try different colors.
TL;DR:
I suggest starting with Spyderco. Many different designs to choose from, most of the "classic" models (delica/endura PM2/3, Manix 2, and Yojimbo 2) have many customizations available, such as Titanium hardware, custom scales, deep carry or unique design clips, backspacers, etc.
Examples:
Sprint Run Spyderco Manix 2, CPM-XHP, desert Ironwood scales, MXG deep carry titanium clip.
Standard S30V Spyderco Para-Military 2, burlap micarta scales from Ripps Garage Tech, MXG gear deep carry clip, random black screws from etsy, titanium lanyard hole.