Haddock is a mild, substantial white fish.
Sea Bass is good and mild but I'm not sure what the availability is nowadays.
Flounder is mild but a little "fishier" than the above.
Tilapia is a good "starter" fish, LOL.
For a while I ate Orange Roughy 2x/week. It's a light, sweet, absolutely delicious fish. However, I learned it has a lot mercury and should be eaten no more than 1x/month. So, I avoid it now.
BTW, I love Fox Point seasoning from Penzey's Spices. It's been great on all the above fish. I brush olive oil on the fish and sprinkle Fox Point on top and bake.