At the end of the day it comes down to how good you know your tool, and what you're using it for. And also it isn't just synths. EFFECTS play a major role in shaping your sound. Actually the majority of commercial synths just have a tone of built in effects. And now I'll list some free, pretty freaking good synths, that no one has listed yet.
1) Charllaten - probably spelled that wrong. It's a subtractive synth. The interface is amazingly simple. Usually use that for my leads.
2) Basic65- A 'chiptune' era sounding vst with a tone of advanced stuff you can do to modulate the sound. It has lfo mod, arpeggio, portamento, delay, pwm, ring mod, and you can modulate a lot of parameters using lfo-including ADSR and all of the effects I mentioned. Then it has a 'dirt' knob which really puts the sound through some kind of bitcrush effect, making it sound 'old'. Also to top the cake there's chorus, unison, flanger, and a gater. Plus prob I forgot to mention some other stuff.
Synth1) Someone already mentioned. But yea.
Oatmeal)This thing is a monster.
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Check out De La Mancha, and DSK. They make all kinds of free synths.
Stock plugins. Get to know your stock plugins.