n13L5My favorite would be a PC-synth capable of using my Android phone or Tablet as a realtime input!!
If it can be an iOS device it's already very possible. I use my iPad with Lemur all the time to control various synths. It's a great partner to Reaktor for weird stuff. Since you seem to enjoy a wide variety of sounds, including acoustics, I would suggest NI Komplete or Spectrasonics Omnisphere for a real upgrade. Neither of them is super simple to program but since you seem to be undecided on a specific type of synthesis, this is a problem you'll run into anyway - needing to learn many different parameters and screens. I suppose Omnisphere at least sticks to one "engine".
EDIT:
Let me amend that. That would be my recommendation for gaining access to a wide variety of sounds, which I took from your statement about not really having a preference for any specific sound. Both Komplete and Omnisphere are kinda "catch-alls" in that sense. If you are talking about an interface that invites you to tweak and make your own sound, I will have to go with a "knobby" hardware synth any day. I can heartily recommend something like the reissued MS20-mini from Korg, a Sub Phatty from Moog or a Bass Station 2 from Novation. All of these are true analog subtractive synths, meaning they're what nearly all virtual analog are designed to copy. This means they lack in convenience in the sense that they don't save their preset inside your project, etc. but they make up for it in sound quality and "fun to play". If you enjoy synth sounds, that is, cause you will never get a realistic piano or violin from them, plus the ones I mentioned are all monophonic. Honestly the very best option to learn synthesis on right now would be an MS20 mini, as it is true one-knob-per-function. It doesn't have menu's because it has no memory. The sound is what you set the knobs to. While this is limiting, it will also exponentially increase your understanding of other synths, including soft synths, most of which are based around some kind of subtractive synthesis.