Home Assistant is out top favorite in this category. With more that 1400 (and growing) integrations it can work with most/all your existing devices and create cross-vendor automatons that no other product can.
I need to warn you – the learning process is significant (it was at least for me), but the documentation is well maintained and the community is huge.
Here’s my current HA dashboard, just to get a glimpse of what this thing can do.
