My favorite lake in the Midwest is Storm Lake in Iowa, just because of how incredibly random it is surrounded by flat farmland.
Not the most original choice but being a yooper I’m biased towards Lake Superior. It’s cold to swim in but you get used to it and it’s always clean and clear. There’s a lot of nice beaches in Marquette too.
There’s a Superior beach in the Keweenaw peninsula in particular that’s my favorite though. It has beautiful “singing sands” and is shallow so it’s usually warmer water and has a really nice turquoise hue, almost Caribbean-like. And hardly anyone is ever there, only locals really know about it.