Your playstyle and build dictate the "best" shield for you, there is no single answer.
Lower armor/higher hitpoint shields work better against shield breaking weapons and projectiles. A higher resistance shield is better vs non-shield-breaking melee weapons.
If you have 4-5 shield skill, pick something fast. If you are going 7+ shield skill, you can use the slowest shields and still block reliably. Using the fastest shields at higher shield skill is overkill, your enemy's swings and your reflexes won't be able to use the "extra" speed in most cases.
I had a +3 steel shield when I played a gen of 1h/shield infantry with 21/18 stats. It felt slow, but you could count on it to catch arrows until end of round/your death, no matter what. Even the strongest axes would take 4 or more hits to shatter it.
The best jack of all trades shield is the elite cav, for sure. Find out what you want to specialize in (if at all) shielding against and pick accordingly. Buy a bunch of different shields and try them until one feels right.