I love Sigma for it's HSM, huge range of lenses - there must be one that suits your need.
I love Tokina for it's built quality, solid feel and value for money. Also, the focus ring clutch mechanism is
And what about Tamron - if you hate weight, you would love Tamron. But I dislike the fact that their lenses' AF ring turns in action.
These are just my opinions:
Build
Tokina>sigma>tamron
Lens performance
Tamron>sigma>tokina
AF speed
Sigma>Tamron=Tokina
TOKINA 28-80mm f2.8
SIGMA 24-70mm f2.8 HSM
TAMRON 28-75mm f2.8