Tried the Cailler milk chocolate but wasn’t a big fan. However I usually buy dark chocolate. At Co-op lot of different bars and wondered which are best.
I think you're in Lucerne, so you can also go to Läderach (1 in train station, 1 in town) or the local Max Chocolatier. Bachmann (the stores all over town with pink signs) is also local and they make decent chocolates too. Any of these 3 are a bit more "special" than what you find at Coop.
If you want to stick to the supermarket brands, the big brands are Lindt, Cailler, etc. To me they are all good, but kind of the same. At Migros they stock Frey chocolate, which a lot of people like (also served on board Swiss airplanes. To be honest, even the Coop store brand is decent for "everyday" chocolate.
My favorite Swiss chocolate is Villars and comes in dark. Although not so easy to find. I've seen a limited variety at coop in the past, however, at a Swiss highway stop this past May, hit the mother lode 😆, came home with many different kinds (dark chocolate bars as well as filled with different types of spirits).
The frey that hopper mentioned is one of my favorites. You can get a variety bag with minis. Are you going to be in Zurich at all? Sprungli is good. Oh you know what, you can get Sprungli at the Zurich airport. Ok, now I’m craving chocolate and all mine from my June trip is gone already. ☹
and nearly right next to the Zürcher Kantonalbank (and its fabulous Rhino) in heart of the financial district is the fabulous Confiserie Sprüngli Café upstairs at Bahnhofstrasse 21. Yum. It was a bit crowded last week - as the summer builds (if the numbers don't get to first) it will build.... as I said, Yum.
Läderach is very good. There are multiple locations in Zurich, including the airport.