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Reading around the boards, I see that people often like some rules from one system and other rules from others.
For example, Bolt Action turn activations and old skool Necromunda second edition\ShadowWar style cover save and movement mechanics, or some other mix like this.

So, being that I have zero experience in most other rules systems, I wonder what you consider the best of possible systems for 40k.
What mix of rules and models would add up to the perfect game for you?
Or maybe someone has already got everything just right, and you could recommend a rules system besides 40k?

Bonus points if these are OOP or free and so the parts can be assembled into an open source community supported modular system from which people can pick and choose to build their own house game systems, all from the same set of sources. What I have in mind here is a sort of catalogue of mechanics governing the various modes of on table interaction, that might eventually become something like a 41st Age system.

You already mentioned Bolt Action, and I think Beyond the Gate of Antares uses a similar system (with D10's instead of D6's). That would be the go-to for revising 40K. You would just need a system for handling Psychic orders.

For small skirmishes, some of the rules from Burning of Prospero would be interesting - the 2E idea of different-sized dice for attacks and defenses. It'd be annoying building/rebuilding dice pools with lots of models, but with about 10-15 infantry, would probably be fun.