Calangute may be commercial, but it has a friendly vibe, and feels quite safe if you are travelling around alone. There are some superb eateries, like Hotel Clisher, and the Tibetan Restaurant, and the Plantain. And has good bus links to Mapsua and surrounding areas.
Plus it is a walk down the beach to Candalem and up to Baga. Although be wary of using the beach at night, as lots of men sleep there at night. And also, do not get called into the dunes, stay on the beaten track.
If you want to meet up with the hippie community, head to Baga beach for sunset.
Candolem is full of ex-pats, but beware because the prices go up too.
I never got to the Southern beaches, but I did do the Spice Plantation tour, and the waterfall & monkey tour. I would recommend the Spice Tour, but not the latter.
Whilst Goa is okay, I wouldn't say they are the best beaches in the world. Southern Thailand, and Cambodia have some incredibly beautiful places, and the people there, do not want to have photographs taken with you, nor do they want to touch you all the time...