Our system is just plain quick and dirty, but it has served us well for a very long time and is the preferred method compared to all of the other systems we have tried.
Here it is:
1. Declare action, includes spells, OB/DB, movement etc.
2. Roll 1 d10 (the random factor)
3. Add your initiative bonuses, including:
Quickness bonus / 5 (round up)
+20 (instant spell), +12 Movement, +10 Static Action, +8 Spell casting, +4 Missile, +0 Melee
Add in weapon bonuses (we have a full list)
Subtract Armour penalty (Again, we have a full list)
4. Each of my players writes down their initiative on our playing board
5. Go around the table in order. Player initiative always trumps NPC and int he result fo a player tie, we go around the table clock-wise.
Quite often players will list the base initiative next to their OBs.
There are cases in which players may want to do things that result in different initiatives, with the exception of instantaneous spells (in which case they get 2 initiative scores to act upon) the lowest is used.
Note: Sometimes it can be quite interesting to see the spell user unable to get off his instant defence spells in time.