Measured by time playing, Logic's synths are what I play the most - particularly ES2, Sculpture, and EXS24.
However, if I was picking one favorite for a brand new user starting from scratch, I'd probably pick Zeeon for iOS. I chose it because:
the UI is a joy to use. Exactly what it needs to have and nothing more.
the patches are really expressive and they all sound fantastic. I feel like some synths (like Synth One for iOS) are intended for more complex patches that don't lend themselves to being played. Those patches already have so much happening that the player's expression doesn't stand out.
it requires almost zero configuration. I usually just map expression/CC11 to filter cutoff, which takes 10 seconds (
like this).
the default patches include a pretty wide range (and
third-party patch banks). Probably the weakest category is bowed instruments.
it's $10 :-)
Happy playing,
Troy