Ao Nang beach is not all that nice compared to West Railay and nearby (can walk in under 15 minutes - no hills) Phra Nang. Actually the part of Ao Nang in front of the town is less than whelming - gets better way down the east end (technically more the north-east end) against the headland.
Ton Sai beach looks sweet high tide but turns to dirty sand, exposed rocks low tide. But you can scramble over the steep headland to lovely West Railay in 10 minutes (although lots of people wade around the headland at lower tides - but rough underfoot for me). There is also a high saddle track joining Ton Sai with East Raily - maybe 30 minutes, steep in parts.
East Railay beach is daggy - lotsa mangroves along 60% at high tide, extensive mud-flats at low tide. But it's a good place to stay - accomm in all price ranges and less than 10 minutes to both West Railay and PhraNang.
Nightlife Ton Sai will have the most variety but there are enough bars around East Railay and Ton Sai to have a good time.
Other activities - all the daytrips are equally offered at the Railays and Ton Sai that you can do out of Ao Nang. But snorkelling off the beach is not much at Ao Nang (not super off Railay/Ton Sai either) and you can't climb or like me, watch the climbers at Ao Nang. Not too many hiking trails/viewpoints at Ao Nang either. Kayaking is popular off West Railay - no doubt you could do it off Ao Nang but I haven't seen kayaks for hire lined up on the beach.
I've not seen a beach party at Ao Nang but towards sunset the beach mats and candles usually come out on West Railay. It's nice at that time - most of the daytrippers from Ao Nang and Krabi town have gone home. Later on the floodlights highlighting the awesome karst headlands come on.
The beach bar at Ton Sai resort has a nice atmosphere around then - lotsa climbers and travellers kicking back and usually someone ont the nearby rockface to check out.
Shopping is way better at Ao Nang. More the necessities at the others although the Railays are beginning to get a few places where you can buy clothers etc.
Me, I always try to stay at Railay/Ton Sai these days.