As stated above, FL Studio has a great arsenal of plugins. Sytrus is a complete powertool, you just need to spend some time understanding its interface. Harmor is great too, especially if you like to experiment as it has some really sweet under-the-hood features. Morphine is a great tool too with lots of possibilities, though it doesn't have that great workflow, so I wouldn't recommend using it for standard synthesis if you want to save time.
But note that both Harmor and Morphine aren't includeed in all FL Studio versions, so you might not actually have them.
Otherwise there are tons of free software synths out there, like U-He Tyrell, Synth1, Eclipsis (haven't actually tried that one). Just head over to the KVR plugin database
But if you want to get a synth with a good amount of presetbanks available, you probably will have to pay around $150 for one or so, as we're talking about Massive, Sylenth1, Serum, Spire, etc. Synth1 seem to have lots of soundbanks too though, but I don't expect that high quality in them though.