Piano.
The inability to mess with the timbre makes me focus on what I’m playing. It’s like blinders on a horse. Also, the ability to play bass and melody simultaneously - and to play in the in-between zones where chords become arpeggios become melodies - and it’s all instantaneous. When an idea is promising, I make a quick recording with a Tascam that sits on the music stand.
There are times when I love messing with envelopes and cross-mod etc for hours. But in the end I get more reward from songwriting than from sound design, and piano’s the tool for that.