does a readied action trump a swift action?
I have a readied action to cast magic missile (standard casting time spell) at my target as soon as he starts casting a spell, I am doing this to interrupt the targets spell casting.
the target knowing about my tactics from past interactions. attempts to cast a quickened Shield spell that he prepared just for this occasion.
does my standard action spell beat his swift action one?
By RAW yes, your readied action happens just before the one that triggers it.
So this just came up in our game -- ranged attacker readied an action to shoot the caster "as soon as he starts casting" and caster tried to cast a Quickened spell.
The rules seem cut-and-dry: Yes, you can interrupt a quickened spell with a readied action.
Does anyone see it any differently?
Nope. The rule is as you and Quatar (back in 2012!) stated it.
I think the idea is that you've taken the standard action previously and it take even less than a swift action to complete (speaking the last syllable of the spell or releasing the drawn arrow).