Which instrument to select on keyboard for ear training?

The one that works for you. Ear training is more efficient when you learn to play songs. Your brain kinda creates an internal MP3 player that recoups the exact frequencies you memorize the most. When you play songs, you not only memorize the sound of your instrument, but also the frequency of the original song you're covering, the sensations you feel like touching the strings of a violin in your orchestra performance for your parents crying proudly behind the 1st chairs... Everything contributes to ear training. The more the frequency gets stuck in your head, the better.

I don't agree "instrument x is better" argument when such instrument is not what you play. Music must touch your heart to get more memorized.

One thing I tell my students (I am a guitar teacher) is if there is a piano in the house, or a guitar on a stand handy, when you walk by lets say the piano (which is way better than a synth sound on the computer as it is a clear tone) hit a random note, and try to match it with your voice.

It can be any note, just pluck a string on a guitar or piano as you are walking by, and sing the note.

You can do the same thing on the computer with a midi keyboard. Piano is the best as mentioned before. Random key... then sing it. It really trains your ear and is easy to do. Just keep doing it everyday.

Personally, I would avoid too "synthy" sounds with swirling tone changes and very plain tones like a sine wave. At least for me I have trouble telling the difference of intervals from a plain sine wave.

Sounds like plain piano or harpsichord should work fine. For sustained sound strings, wind, or voice should be fine, but @chasly makes a good point about vibrato. You may want to avoid too much.

For ear training resources the two things I think helped me - and I need all the help I can get - were singing along to the chords of the rule of the octave and singing along to simple songs while strumming guitar chords. I often used folk song books, because the material was fairly simple and short. The point wasn't to practice the songs for an audience. It was all about singing the melody tones and listening to my voice get in tune with the guitar chords. Importantly, this trains your ear to real musical settings.

Ear training programs I tried just didn't help me. Too much randomness from chord to chord or interval to interval. Maybe they pay off with a lot of dedicated practice, but I preferred something more practical.

There are several apps that you can use for ear training, below mentioned can be most effective

1. Tenuto
2. Teoria

There is an article all about ear training that will be helpful, you can check it out here: https://blog.landr.com/best-ear-training-apps/

hope this helps!

If you choose a pipe organ-stop (i.e. not a reed), then you will get the purest note with no vibrato.

On a violin for example it is difficult to pitch notes perfectly and so beginners can cheat by using vibrato. This means that they are on the right note some of the time. It's easier to get away with being off-pitch whether you are singing or listening if there is vibrato on the note.

If you want to train your ear it depends in what you want to achieve. There are two separate accomplishments (1) relative pitch and (2) perfect pitch.

All musicians need some ability with relative pitch unless they simply play from the page all the time and don't really listen to themselves. Perfect pitch is difficult but not impossible to learn. Just like learning a new language, it gets harder as you get older.

Singing is always a good way to go.

To practice perfect pitch, you can sing a note and then see if you can find it on the keyboard. Pick random notes and with practice you may be able to go straight to the correct note without hunting around. Note that some people never manage this while a few seem to be born with the ability. Don't worry, you can become an excellent musician without it.

To practise relative pitch, pick a note on the keyboard, play it and sing that note. Then sing a melody to yourself starting on that note. It can be a popular song for example. If you can't remember the tune well enough to sing it then pick one you do know. Next see if you can pick out the tune with one finger on the keyboard as you sing along. It helps if you know your scales in different keys because this will give you a clue which notes to try.

The rule is little and often, 5 minutes a day is better than 2 hours every 2 weeks. The reason is that your brain organises each day's knowledge when you sleep. Your brain will also learn that this must be important to you because you do it so often.

Make up your own games by playing first and singing after or singing first and playing after. Finally sing and play simultaneously.

The last technique is used a lot by jazz musicians because it helps them to improvise anything they could sing. Some classical musicians do it and on some older recordings you can actually hear them humming along.