Sylenth1 is my go-to synth for sounds, but it doesn't cover everything I need. I use Harmor for more complex, digital sounds and textures, for example.
I think it's a good idea to get two or three quite-different synthesizers to cover all your bases. Sylenth1 does subtractive, rich 'bread & butter' sounds perfectly, then Harmor does the crazier, more experimental things like Resynthesis. I have Massive for Wavetable-style synthesis too (and Z3TA 2, when it f***ing works), and that seems to cover every type of sound I'd like to design.
I don't know how detailed Serum is, and there's UVI's Falcon and a future WT one coming from Tone2, but I think it's very difficult to get one synth that does absolutely everything. I'd love to be proven wrong though!