My first combat-capable missile guided completely by Funky trees! Is uses only wing parts and rotators for control, a custom detonator as a warhead, an engine for propulsion and a rocket part for the smoke trail. Does not use stock missiles and PID controllers.

ST-1 Wayfarer is a large surface-to-air interception missile. It is relatively precise and maneuverable, although there is a lot of room for improvement (for example, it cannot operate in very close ranges).
To launch, choose the target and press AG1, use high physics setting

How does it work?

Instead of using built-in variables for target heading and elevation, the guidance system first determines the 3D coordinates of the target (code for those here, credit to @bjac0). While more complicated, this makes leading the target easier: the final target coordinates are the sum of current coordinates and their rates of change multiplied by time to impact. The heading and elevation are then calculated manually, multiplied by their rates of change (useful for sharper turns), smoothed by their absolute values to reduce wobble and passed to control rotators.
The detonator activates when two conditions are met: the missile is close to the target and the derivative of distance to target is positive. This means it always detonates in the closest point to target.

There are some issues but i find the missile quite reliable. If you experience issues or have ideas for improvement, share them in the comments.

Also big thanks to @Ku for popularising FT missiles, check out his works!