Depends on whether you want a Sandy beach or a pebble one!
Kassiopi is great with a busy harbour and lovely shops, bars and restaurants. Pebbly beaches do great snorkelling and there's usually one of the big inflatable water sports things on one of the beaches. It's bustling rather than party central with a good mix of nationalities.
Sidari has all the water sports you could wish for and a glorious Sandy beach but is a bit party central and British. There is the Sidari watersports hotel which is on the outskirts of resort so a bit quieter and it has 3 or 4 slides. We've visited for a day as you can buy a pass which was cheap for a day out somewhere different - bus stop just outside.
Paleokastritsa is beautiful abd where the trendy ones go! Def worth a visit though as it's so stunning. Lots of boat exploration around the coves sbd caves though!
Arillas is lovely but might be a bit quiet for teens. I've not been properly down in the south for years but might be a bit quiet for teens - except Kavos which is party central! I don't like the beach at Ipsos (another busyish resort) either as right by the road! You could look at Be it's, Messonghi, Agios Gordios and St George's south too.
We stay in Roda which we love - plenty to do but not party central. There's no water sports any more on the beach though but you can hire boats. You can get a bus to Kassiopi or Sidari and it's not expensive in a taxi if you want to have some fun on the watersports.
Lots of trips from Roda too and you can go up into the mountains or into town easily. If you want a self catering place which is busy with families (other teens), a swim up bar etc look at Theodoros resort, a quiet place I'd look at Elizabeth apartments and a budget one look at Valentinos. All very close to each other, couple of minutes walk from the beach etc and we've used them all! There also Makis apartments right on the beach. Book early if you need a 2 bedroom apartment and I would book direct with the owner as it's cheaper and you usually get air con and a safe free. You can use any pool as long as you buy a drink and sun loungers are free on the beach if you buy food/drink.
Lovely restaurants along the beach and it's cheaper than a lot of other resorts. My teens go back there with their pals now as they're not into lager lout places! There are a couple of bars with tribute acts and a couple of others who do quizzes etc but no ott carry on! Good mix of nationalities.