I initially shared that opinion of Omnisphere, because it was loaded with presets that were designed to sound good in solo - meant to impress would-be buyers trying it out for the first time. Which it does. It truly does. But as you try to use it in mixes, it's often too fat and wide to squeeze in.
Over time, however, I've learned how to put it on a diet so it doesn't take up 3 seats on the airplane. Now it finds its way into almost everything I do.
Impressive though it is, it's definitely not the one synth to rule them all. Think of it as an add-on, after you've already assembled your bread 'n butter sound set.
Not long ago I completed my own personal survey of the state of soft synths in the world, demoing purt-near every product out there that had a demo. Diva impressed me most, but it's a CPU-muncher. Better have a fast machine, and don't plan on making songs with 10 instances of Diva. Second-favorite, and much more CPU-friendly, was Zebra, by the same author. I was able to run 10 instances easily on my elderly machine.
An important part of the equation is how much you like
programming synthesizers. IOW, a) is it easy to program and b) are there lots of presets, either because you don't like programming or to provide a starting point for tweaking.
Zebra, for example, is not particularly friendly for programming (e.g. no way to see all the module interconnections at a glance) but comes with a lot of useful presets, and there are many third-party presets for free or purchase.
If you want something that's easy to program and capable of those classic sounds, consider one of the many MiniMoog emulations. The free (!)
MiniMogue would be a good place to start. To me, it sounds like a pretty respectable clone. There are two main versions, one that's true to the original and one that adds new features such independently-pannable oscillators.