I start by using plain text Markdown files so that I can use many different programs, workflows, and it is future proof.
What do I consider Notes? Documentation, reference materials, ideas, journaling, and task management.
For documentation I use a programmers text editor. Mostly VSCode, but I still use BBEdit, Sublime and others
For the past couple of years the primary application I use for everything else is NotePlan
https://noteplan.co
NotePlan stores its data as markdown files, offers a very polished GUI for Mac, iPhone and iPad and is tailored to each. A web version was recently released and is still feature incomplete but very usable.
The developer has created an app that appears deceptively simple, but has a great deal of depth and flexibility for you to create your own systems.
If you want Tasks and Calendars as part of your note system, NotePlan is well worth a deep review. Check out the videos on their site to get an understanding. NotePlan also integrated with Apple’s Reminders and Calendar. It also can sync with Google Calendar.
Hashtags and @Mentions are available. Templates for meetings, journaling and more are provided or you can make your own.
Sync works flawlessly via Apple’s ICloud and the developer never sees your data. (I was skeptical of this but found it to be true). It is not recommended, but you can also sync via Dropbox or git.
NotePlan is extendable via JavaScript plugins many of which are provided by the community. These include simple shortcuts to complex new dashboard views.
Do you have your own additions to Markdown syntax? NotePlan made it easy for me to add my own syntax.
Since this is Markdown, you can use other tools. The NotePlan documentation explains how to use Obsidian
I mention the Web version is not yet full featured. However, it is VERY functional. My primary workstation is KDE/Ubuntu, and I use the web version all day long.
Finally, yes NotePlan is a subscription. I am hesitant to recommend subscriptions. But let me strongly state, I feel it is a fair deal. The subscription covers 4 native platforms. This is a highly responsive Developer on Discord and elsewhere, new features arrive constantly. I am on the BETA and get updated several times per month. The Developer has gone out of their way to not lockup your data in a proprietary format. The only lock in they have is providing an excellent UI and nurturing a great community.