I'd stay away from soft drinks altogether.
The sugary ones are bad, and there are many concerns about the "diet" additives used in the low calorie ones. I just guzzle water all day.

My successful fitness regimen from 2010/2011:
Get ill and be in a lot of pain for three months.
The nausea from the pain - and the pain medicine - makes eating anything unattractive.
I lost fifty pounds with no exercise! Only gained back ten so far. My clothes hang.
It's still sort of hard to walk, but that's permanent so I don't worry about it.
Stopped the meds after four months (they begin to make thinking very difficult), but didn't get fully back into the eating habit, which I never liked that much anyway.