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This would have been fine to ask on the precure thread on /a/.
You can start just about anywhere, all the series are self contained stories except Max heart (2nd series, continuation of 1st) and GoGo (5th series, continuation of 3rd).
The top recommendations are usually HeartCatch, Smile, Go Princess, or whatever is currently airing. The 22nd season starts next month, so if you wanted to watch it weekly now is a great time to start.
Aside from that, you could just pick whichever cure you like the most and watch her series.
