Falcon and Halion 7 are the two best synths I have in terms of horsepower and just plain utility. They both sound great, although I am inclined to rate Halion slightly higher (and on par with Reaktor) and call it the GOAT in terms of sonic quality.
They are both basically not even synths but “platforms” that allow the user to create complete instruments. Halion is THE best sampler I have ever used (for the process of actually recording in samples that is). Falcon is great in that regard too.
Although I have a ridiculous number of synths, and I highly appreciate and acknowledge mister Eric Persing as one of the most important synth pioneers in history, I have never been tempted by Omnisphere. I know it sounds great, but the UI has never really enticed me to program my own things with it. This is not a dig; the fact that
@pendle likes it and occassionally makes soundsets for it, speaks volumes. Omni and I just don’t seem to click.
So, to the OP: if you’re used to Halion and your aim is to create your own custom synth(s), the choice is easy: Falcon. Have a look at some of Simon Stockhausen’s creations for it, his soundsets make use of ALL synth engines Falcon offers, contain unique high quality samples out of his own personal collection and -importantly- are documented REALLY well. I recommend you buy one of the recent Falcon Singles over at Patchpool just to get a feel for what’s possible with Falcon in terms of creating one’s own creative sound design environments / instruments.