If there could be only one...
Tyros 4 would be it. As a player since the sixties, I've seen most of the evolution of electronic keyboards. I still have most of the good stuff from Arp, MOOG, Sequential Circuits, Roland and Yamaha, as well as rooms full of general studio gear. The T4 replaces it all and more.
The sounds are incredibly realistic. Guitars that scream and whisper. Breathy Sax will let you break their hearts. The default mixes are spot on. I can go from 0 to a finished 44.1k .wav in less than an hour inside the T4 and never leave the digital domain. I've had mine about 2 years and still haven't hit all the features.
Yes, there's a learning curve, but if an old guy like me with all my baggage can grok the T4, anyone can.
Yes, it's pricey, but you get what you pay for. How much would you pay for a real Iron Man suit? This is my Iron Man suit.