I consider the intake opening and the exhaust opening points to be so close in equal importance, I do not know which is MORE important.
I would respectfully disagree with Harold.
The most important valve event is intake close. It will determine compression pressure and reversionary flow. The exact close angle will depend on induction tract pressure and cylinder pressure due to piston positon.
The second most important is exhaust valve open. If the exhaust valve opens too late, the efficiency of the blowdown period is compromised. If it opens too early, the blowdown is not improved but there could be some loss of combustion pressure on the piston.
Next in importance would be the intake valve open since it is the beginning of the overlap period. If the intake opens too early, we again have reversionary flow with exhaust gases backing into the inlet and a resulting dilution of charge. If it is too late, we will not obtain sufficient intake valve lift at the higher piston velocities.
The least important is exhaust valve close. It should be selected to keep the overlap period as short as possible but still evacuate burned gases.
The overall duration is a function of engine size and speed. Imagine the air as a string of pearls, attached to the piston crown. But the string is a rubber band. As the piston moves down, the rubber band stretches. After a while the rubber band contracts and tries to catch up with the piston, bringing the pearls into the cylinder. Even after the piston has reversed direction, the tension in the rubber band will continue to move pearls into the cylinder. Up to a point.
Larger engines have a longer string of pearls to drag into the cylinder. High reving engines have less time for the rubber band to stretch and contract. Remember the toy called a "Slinky". It was a tightly coiled wire that would move from a high position to a low position. By moving your hands up and down, you could make the wires move back and forth.
A major problem is that air is both stretchable and compressible. When valve events are adjusted for one engine speed, it may compromise another. You can't have your cake and eat it too. Unless...
...you use adjustable cam timing, adjustable inlet trumpets, adjustable valve lift, variable boost, etc.
A major consideration in the selection of valve events are pressure ratios; Pi/Pe (inlet pressure divided by exhaust pressure). Any gas will flow from a higher pressure to a lower pressure. That is what resonant tuning is all about.
When the exhaust valve opens, the blowdown gases exit at high pressure and travel down the exhaust pipe. But during overlap (180degs later) we would like a negative exhaust pressure to assist the scavenging process.
At inlet valve open, we would like a high positive pressure at the valve to prevent overlap reversion. At inlet close (270deg later) we would also like a high positive pressure to prevent reversionary flow.
The exhaust and intake tracts operate on different durations, different temperatures and different pressures.
The very best way to select valve events is to use inlet and exhaust pressure sensing vs crankangle on a running engine. Clint can elaborate on this topic in great depth, I'm sure.
An excellent treatise on the topic was written by T.W. Asmus, Chrysler Corp, and is available from SAE as #820749, Valve events and engine operation.