I'll state the obvious;
Em on guitar and C major, Am, and Dm on keys ..
go figger

These are just naturally easiest.
Like Bb concert on trumpet .. everything transposes in my mind from there. It's how I learned.

"But alla us guitarists are good, ain't we?"
No, I been called a guitarist and I ain't good. I'm barely serviceable.
I can think of others.

/that was meant to be funny

I do not work here, but the benefits are still awesome
Make your sound your own!