I see that I regularly get the same questions, like „which is the best synth of these three”, or „which synth should I use to make this sound”.
What you’re about to read down below should answer any
(or the majority of) questions you have to me: 

1.The Best Synth - understanding your needs and your inner sensitivity / sensibility.

Before you ask anybody about the best synth, try to ask about the best knife. First problem you face is… who to ask? A housewife, a butcher,
a sadistic murderer
, a knife manufacturer, or a knife collector? The housewife will tell you that the best knife is the one that prevents butter from falling off its edge. The butcher will show you some large knife that easily cuts through meat and bone. The sadistic murderer likes his crescent-shaped knife to inflict as much pain as possible. And the manufacturer will tell you that their best knife is the most expensive one. The same is with any other item, including synths. So if you ask me which synth is the best synth, I need to ask you what you want to use it for. Practically speaking EVERY synth is the best synth. It just needs to get through to its "best" owner / user, strike a chord with his/her inner sensibility and inspire that person to make music.
Oh, and the collector? He's bound to tell you that he appreciates each of his daggers - that's why he keeps them all, even though he's rarely making any practical use of them. Which brings about the realization that he might be the worst person to come to with a question like that...

 2.The Best Engine and Sound / knowing the natural tone and the possible pointlessness of a vast engine.

I don’t think there is any synth whose sound is always the best sound. Again, it's like asking which drink has the best taste. To give an answer to your question, I really recommend playing the synths personally (tasting all the drinks), or at least listening to them as much as possible in various ways (although consistency is needed; if you test a Blofeld in a shop and then go and "test" a Nord Lead in a Youtube video and come to the conclusion that the latter lacks highs, you're not a noob - you're fucking stupid). When you've been running, you drink cold soda. When you've been out on the ski slope, you drink hot tea - you're optimally satisfying your needs. It's simple as that. Let me continue with food analogy: there's a couple of books about subconscious judgments.  I'll re-tell you, from memory, some of the things I remember:

People were tested for Coke and Pepsi. They were given sips of both drinks and were asked to point at the cup that tasted better. They pointed at Pepsi. So the bozos at Coke frantically changed the Coke formula to make it sweeter to resemble the Pepsi taste more. They introduced the New Coke to the market and... it was a gigantic flop. Why? Because taking one sip of something sweet is perfectly okay. But drinking an entire can is too much. The truth was that people chose Pepsi when taking sips, but actually the market statistics showed that Coke was selling better than Pepsi. Pepsi won only in the artificial sip tests, not in real life. When you have to drink an entire can every day, that is, when you have to spend a lot of time with your synth and make music with it, you really need to have the right thing that will feel good all the time, not the one that seems cool during 10 minutes of random use or during someone else's performance.

Next: people were asked to pick one jam from a collections of various jams. And so they did. But when they were asked to describe the jam's characteristic, or say WHY they liked them, they were helpless. They were no longer certain about their choices. They did not understand what it means for a jam to be slippery or have a texture. Trying to explain your feelings about why you think a given thing is cool, rationalize its coolness so to speak, may result in fucking your mind apart. "I love this synth because it has a big LCD"... ok, great, who the fuck cares about LCDs? (I do, but that's irrelevant). Or: "I love this synth because it just sounds so lush"... oh yeah? What does "lush" mean at all? We all know for ourselves what a tasty jam is, or what an enjoyable synth is. "You know what you are, I know what I am, we can only judge ourselves", as someone once said. There are people who say that the Virus sounds analog and the Waldorf Q sounds sterile, I and others say the opposite. There are people who say the JP-8000 sounds terribly plastic, whereas I and others don't agree. We're in a void here. All is relative. Thanks, Einstein.

Having a deep synth engine is great, but equally important to that is the natural (inherent) tone. A deep engine can give you thousands of various patches, and patches are translation of yourself and make up the ultimate vibe that you want to achieve in music, but in my opinion the foremost aspect of a synth that you should consider is this overall, permanent tone. Because there's little use of a monster synth whose natural sound puts you off. So try and get to know the inborn, inescapable sound of the synth and decide if you like it or not. You won’t be digging much in the engine if you constantly get irritated by the flatness / bigness / coldness / etc of the sound, and by the inability to achieve your sound. It's the 21st century and all synths now have all the elements like FM, LFOs and Envelopes that are needed to make this or that kind of patch. "Can this synth make pads?" Of course it fucking can, it has the Attack and the Release setting which you can set at high values. The question you should focus at instead is: "do the pads sound convincing, and are they in any way special to me?

3.The Best Interface / driving down the highway or crashing on every tree.

Don't forget the interface. Assume you have a synth that sounds great and has the deepest engine out there, but its interface and editing are tedious and frustrating, and you don't have all the fucking time in the world. For how long are you gonna get along with such a synth? It’s like driving a Toyota down a highway, or, on the other hand, driving Bond's V8 Vantage-Volante which has joysticks instead of the steering wheel, and will start only if you activate the windscreen wipers first. If the interface / software on a synth has some serious quirks, it can seriously undermine your experience. Even one tiny thing like encoders instead of knobs can render your synth frustrating and soon gotten rid of. And of course you won’t be getting all of the power, or all of it quick with an awkward interface, so basically you’ve wasted a portion of your money. If the same kind of weapon exists in two types, don't hesistate to pay good money for good interface, because what you're doing by that, is you're buying off the wasted future time. And that time is your life.

."Why do all your demos sound the same?"

Well, guess what? Guess who's making them?
Besides, if it is true, does it say more about me, or maybe about the synths themselves and their alleged differences in sound that are sooooo inimitable ?