This was the first MIDI application that just worked for me straight away. I could just plug in my MIDI keyboard and start playing a few notes. What's more, it's one of the few that features a light theme by default, so thanks for being sensible and making the user interface more accessible too.
This soft synth is derived from ZynAddSubFX, and is similar, but has some differences. They're both very powerful. ZynAddSubFX runs on Linux, MacOS, and uh, what's that other operating system? I can't remember its name; a lot of people are forced to use it. So if multiple platform runability is important to you, you will want to go with ZynAddSubFX. But if you want something specifically optimized for Linux (Yay!!) then you'll want to use Yoshimi. That's what I use. See my review of ZynAddSubFX to learn how I got both of them to work with my external MIDI keyboard using the aconnect command.
It plays sounds out of the box on my laptop, both with my laptop keyboard and with a virtual keyboard