Lets see from worst to best in terms of performance.

(8 speed)
Shimano 2300

(9 speed)
Shimano Sora
Shimano Tiagra

(10/11 speed)
Shimano 105
Shimano Ultegra = SRAM Rival
Shimano Dura-Ace = SRAM Force = Campagnolo Record
SRAM Red = Campagnolo Super Record
Shimano DI2

Shimano DI2 is the best. It's hard to beat electronic components. It's also really pricey though.
I'd say SRAM Red is the best mechanical. Very lightweight and fast. I've also heard Super Record is very smooth. Oh and Dura-Ace could be up their with Red and Super Record.