The original draft of the v2.2 boss theme was done in OpenMPT. I wouldn't call it fully featured or particularly intuitive, but it does pretty much everything I want it to do. It's a tracker module editor, so there's no piano roll, which might be a deal breaker for you. You can create instruments using samples, basic FM synths, or VST plugins. Most VSTs I've thrown at it work, but sometimes it takes extra effort to get things like pitch bending working depending on which plugin you're using. Additionally, you can import soundfonts (.sf2 and .sfz files) which will be added to the quick menu on the left side of the app so that you can quickly and easily preview instruments they contain. You can drag and drop to add that instrument to your project.
If you're just getting started with creating music, OpenMPT could be a good place to start -- you can do a lot with just the basic functionality, and you're not stuck with your computer's MIDI soundfont. Just keep in mind that you will probably need to consult
the manual often until you get used to working with it.
I can't personally recommend anything more advanced than OpenMPT -- I've been writing music for over a decade, and interfaces like FL Studio's are still totally alien to me. (Last time I tried FL, I had to get help from a friend just to figure out how to place one note!) That is to say, I don't have nearly enough experience in more advanced editors to give a solid opinion on them.
If, on the other hand, you're looking for something super basic, you could try a pure MIDI editor such as Sekaiju. I used to use one called Anvil Studio back in the day. It's more limited, but it's decently intuitive in my opinion. I mean, if I could figure out how to use it as a 12-year-old, then I think anyone could learn it well enough to jot down musical ideas.
As an aside, be prepared to deal with writing music you (or others) think is no good. Few people make bangers on their first attempt. I spent at least 2 or 3 years writing total garbage before I picked up how to make a decent piece. Writing music is like any other skill in that you will not be good at it until you practice for long enough.
I hope my rambling is useful to you. I wish you luck on your musical journey.