Dirt Simple Deterministic Group Initiative

Initiative is seized by the group which

1. has magic that lets them act first,
2. springs an ambush (and they also get a surprise round),
3. can see clearly while the others can't,
4. has already drawn their weapons while the others didn't,
5. has weapons of longer reach.

Resolve in the given order.

If none of the above settles initiative, then it's a matter of luck and circumstances. Both sides roll a die and the higher result wins, with ties going to the PCs.
 

Ambush goes before light, so you don't need to
give away your location with torches.
From here.

 
Ambush is the (second) best for securing initiative and winning the fight before it has even begun, so that should push the players to never fight fair. Good.

Number three mostly applies when only one group has a light source or means of magical vision. Light is vital in the dungeons.

Natural weapons and maybe a dagger hidden in your sleeve can be always considered drawn.

Number four together with five lets you rush archers that are not ready to fire, but otherwise you should take cover. Also spears and polearms get a little buff.