Riposte is on the bottom. Then it goes (if my memory is working!) dual parry>dodge>parry>two handed>shield block.
But it isn't linear. The defenses on the weak end are more affected by things like terrain, sight, hitroll, weapon class, weight, etc. But again, dodge will be more affected by terrain and parry by weapon class...so don't just think in terms of x is better than y.
From what I understand, there actually isn't much difference between two handed and shield block except if you are blind or that you can lose two defenses when you lose your weapon with two handed, and only one with shield block.
Basically, if you are outgeared completely, your shield is about the only reliable defense available, and even it won't be very good.
Dual wield has an effect on your parry/hit chance, although this may be completely negated when mastered? You can really notice it at 75%.
From my understanding, and again I may be wrong, but the multiple attack skills have a base chance to fire plus any modifier (such as dual wield, dex). So, for example, if your second attack fires 80% of the time, your third may fire 50% of the times that the 80% succeeds. If you pass both of those checks, you can try another 25% for a fourth attack. I'm sure the numbers for dual wield are lower...
So yes, a warrior has the potential for 8 attacks, but you aren't very likely to get those unless you are feral and/or hasted. 
Feral (mini haste) and normal haste are just modifiers that increase the multiple attack probability and they give one extra base attack.