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.