Slippery Mind, all the way. I don't think we need Elder-Basilisk to come along and give us percentages, but will saves can often be the most deadly in the game (esp if you dont have a good will save!). That extra save is huge.
I agree with others about Imp. Evasion. By the time you get it, you aren't likely to ever fail a reflex save, Imp. Evasion just lets you take half-damage even if you roll a 1 or 2, otherwise its the same old evasion.
Skill Mastery would be next for me, its fairly powerful for rogues with high search scores (minimize search times), or of course for the stand-bys - Move Silently, Hide, Spot, and Listen. Of course it also comes in handy for breaking locks and snapping devices, and for just about anything else the rogue does often.
I think the new Crippling Strike is about tied for Skill Mastery. 2 point of strength damage every sneak attack can add up fast. If you have 2 attacks, thats 4 chances for a TWF or 3 chances for a ranged rogue (assuming they both find a way to sneak) which is about 8 Str, or the difference between an 18 Str fighter and a 10 Str fighter! Either way, after a couple rounds that Str damage will be quite telling to even the strongest monster.
I also like Opportunist, but in play the rogue in our party didn't get to actually use it much. Still, when it does happen, its very nice. I place it below Crippling Strike only because C.S. works EVERY sneak attack, its very dependable.
1. Slippery Mind
2. Skill Mastery
3. Crippling Strike
4. Opportunist
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