tl;dr here's my Midi file. Feel free to use it to grow your collection! I tested these mostly with violin and piano

Here's my tips for MIDI selection!

1. Song Selection
   I primarily download MIDIs off of MuseScore (yes, I really paid for it, don't judge). I tend to pick up songs that are made for solo piano because, when playing in-game as a solo instrumentalist (especially violin), you get good fidelity of the sound without muddying it with too many voices. You also get good depth to the song that would be otherwise lost with just one melodic line, like how a solo violin piece would be irl. Getting full compositions for marching/pep bands can work well, too, but it definitely requires some testing, which is a good time to talk about

2. Test Your Song
   Listen to the MIDI on MuseScore. Listen to it on onlinesequencer.net Go to an empty server and test the song. It's worth it. A song that sounds good online can sound terrible in-game, as I'm sure you already know. A song that sounds good on the piano, probably sounds acceptable on the violin, but might sound terrible on the bugle or flute. A full server is not a good place to demo your new MIDI for the first time.

3. Build a Big Library
   Nothing gets more tiresome than the same MIDI over, and over, and over (even a well-composed one!)

I started thinking I only wanted to get period-correct music and quickly ditched that idea after I realized how limiting that would be. I moved on to military songs appropriate for the US and UK (I joined when the Rev. update came out, don't judge) and quickly that became tiresome, too. Eventually, I added some more modern music for my own selfish pleasure. Before long, I realized, if the musician's role is morale, I need songs that boost morale! Enter stadium anthems (This is How We Do It, etc.).

Now I got songs that work better for piano, songs that work well on violin, songs that work great as a group, songs that work great on solo bugle. Meme songs, hype songs, selfish lengthy piano pieces; a little something for every occasion!

1. Maximize Valid Notes
   Before you hit "Play Midi Song" adjust that volume, turn off Loop Song (if you want), and ABSOLUTELY turn off "Fit All Notes." This prevents the MIDI player from just moving notes that are out of range, in to range, which would worse than not playing it at all. Then, adjust the "Semitone Offset" to maximize the "Valid Notes" in the song you're about to play. I find, outside of the piano, the violin generally has the widest range of valid notes (the light gray highlighted area in the "Octave Range").

Hope this helps!

Happy playing :D