I'd say the best in PURE performance would be the XP-90C, followed by XP-120, then SI-120. The XP-90C would give you the lowest temps possible, but it'll only be around at MOST 0.5-1.5C better then the XP-120, and because of that temp difference, you might as well go with a quieter 120mm fan, the XP-90C's noise/performance ratio isn't too good.
From most reviews, it shows the SI-120 to be 1-2C worse then the XP-120, due to its shape and size. The SI-120 should be able to fit in ALL motherboards because of its height and placement, but the XP-120's fins are a tad lower with more heatpipes, which might interfer with RAM placement or other parts of the motherboard.
If you don't care about noise at all, go with the XP-90C.
If you want something that performs SLIGHTLY worse, but should be much quieter and can cool the other parts of your motherboard due to the larger 120mm fan, go with either the XP-120 or SI-120. Check thermalrights website to see which fits your motherboard.