I have learned to play many instruments, including, clarinet, bass clarinet, sax, flute and piano. If you asked me what was the easiest for me to learn I think it was the flute. I learned this instrument second after the clarinet and my abilities with that were a big boost to learning the flute. What about you?
The piano is probably the easiest to learn for me because all you have to do is know the notes. It's not that difficult compared to other instruments since it's similar to playing the keyboard. Like all others, it's also easy to forget when you don't play regularly.