Where to base? Cassis, La Ciotat or Martigues?

I do not recommend Martigues. The city center is cute in an unpretentious way, but the city is surrounded by oil refineries ruining the surrounding landscape, and you'd have to drive quite a bit to reach most of the sights Provence is famous for.

Between La Ciotat and Cassis, if you think Martigues is your vibe, then La Ciotat will certainly be a match! The hulking presence of the shipyard in town is a slightly gritty backdrop, but, set against the cliffs of Rocher de l'Aigle, it is a unique sight, and there is no noise or pollution (the yard mostly repairs luxury yachts these days). As for the town itself, it is really on the "up": many cute shops have popped up, the restaurant game is getting quite good (do check reviews, there are some tourist traps on the harborfront), there's also some fantastic ice cream shops. And the Mugel & Figuerolles calanques are really pretty!

While I never stayed in La Ciotat (I have access to free accommodation in the region), the Best Western Vieux Port looks like the best base in town. The building is modern, but the location is perfect (in the town center but also walking distance from the Mugel & Figuerolles calanques), there's a rooftop pool, and the prices are reasonable.

Cassis is more polished, more touristy, the setting is a bit more dramatic with 1,000 ft cliffs immediately to the east, but the village is smaller and does not have that much to offer IMO, and it does not feel like a place where people live (at least outside winter).

How long are you staying? The town of Bandol is charming, with a nice little beach, lots of restaurants, a sweet waterfront and marina, close to wineries for tasting the great rose of the area. If you have a car there are some nice drives into the hills or to other beach towns. Sanary-sur-Mer is right next door and another good choice. I don't recommend Martigues or la Ciotat for the reasons stated above. (I am partial to Bandol because we have family there, but it is popular with the French for vacations.

We are staying in La Ciotat for 3 weeks in June after going through this process ourselves. I've heard great things about Cassis also tho!

In addition to my comments on La Ciotat, I certainly second Bandol and Sanary. In fact, Sanary is where my "base" in the area is.
They're a bit further east, though, so whether they're a good call for you also depends on what you want to visit from your base.

Back in 1999 when we checked out of our hotel in Avignon and told desk clerk that we were heading to La Ciotat, her reaction was “Ppppppfffffffhhhhhh, C’est tres industrielle.”

We stayed by the beach at a nice beach hotel on the east end of town about a mile from the very large and visible ship works. Went off to Cassis one night for dinner (by car). Nice dinner in Cassis, views and yes, touristy even in 1999. Nice day on the beach in La Ciotat, but I’m glad we were only there two nights.

I’d think Bandol might be a nice alternative, but I’ve not been there. From La Ciotat, we travelled west to Les Baux de Provence, Nimes and Spain.

Duration and method of transportation matter, but Cassis is lovely. Great, walkable size, charming marina, a couple nice beaches, views, and nature. It has a laid-back holiday feel, lively at times but not oppressive, shops but no tourist stands.

It's probably not the best hub for frequent day trips. Parking may be tricky, and the train station is up the hill and away from the center.

La Ciotat in 2026 is really, really not the same place as in 1999. It was far more industrial back then, and less spruced up.

And Cassis is a lot more crowded than in 1999. We spent six months there in 2000,