Aalto is one of my favorite softsynths, and the one that probably gets the most use. It's like a modernized and slightly expanded version of the Buchla Easel. Personally I could care less about presets, I design sounds from scratch -- so something that is both relatively simple and yet super flexible, like this, is ideal. It also just seems to go with everything.
Kaivo is creative but I generally struggle to find a place for it in my music, so I've used it very little.
I keep meaning to check out Virta because it might be a great way to process my bass guitar, but I keep putting it off.
I'm really looking forward to Sumu -- I've been impressed by what I've seen/heard in the teasers.