Well…
The 1.8 is a fine, fast piece of glass with a ring (not USM) motor and a plastic mount. It has a tiny focusing ring at the front of the barrel for when you want to manually focus.
It has 6 elements in 5 groups, and the aperture has 5 blades.
You can get one at B&H for $120 with free shipping. A bargain (although a mere three years ago it was $75)
The 1.4 is a finer, faster piece of glass with a USM motor and a metal mount. It has 1/4 inch focusing ring mid-barrel.
It has 7 elements in 6 groups, and the aperture has 8 blades.
B&H has this one for $379 with free shipping.
The 1.2L is the top of the line with a USM motor, and I think a metal body. (Not sure; never held one.) Weather-sealed, with 8 elements in 7 groups, but still with 8 blades in the aperture (same as the 1.4)
B&H has a special on this, until Sept. 3rd you can get it for a mere $1499. After Sept. 3rd it goes back to its usual price of $1619. Either way, you have to pay shipping on this baby.
The 1.8 is a fine lens, especially on a budget. I used it happily until I got my 5D; by then I wanted the 1.4, mainly for the better build quality and the better bokah. (Bokah - out-of-focus background. More blades=rounder aperture=better bokah.)
Given that the T2i has an 18 megapixel sensor, I’d recommend the 1.4, if you can afford it. The higher quality glass will resolve the image better on a sensor with such tiny pixels than the 1.8 will (IMHO). (Declan’s 30D has 8.2 MP)
But if it’s not in your budget, go for the 1.8.
As for the 1.2L, that seems to me to be a lens they make because they can, and because some people have more money for glass than practical applications.