I am afraid my list will be mostly useless for non-Indians and redundant/contentious for Indians.

My favorites are the ones that require either a near-native comprehension of Hindi (because much of the value is in dialogue nuances that will be lost in subtitles) OR movies that require a fairly deep understanding of the Indian cultural context.

Also, you cannot really watch only the "good" movies. You kinda have to educate yourself by watching movies that I don't personally think are very good, but were really influential and shaped the context within which other movies can be understood and appreciated. It's like you cannot appreciate certain jokes in The Simpsons without appreciating broader stuff from American culture, including bad Arnold Schwarzenneger movies (to understand the Rainier Wolfcastle jokes). The Simpsons is worth watching, most Arnie movies are not.

If you really want to get into Bollywood (I am not sure why you would want to, unless you have a curiosity or practical reason to learn about Indian culture, with Bollywood serving as cultural texts to be interpreted rather than entertainment), there is plenty of good stuff on Wikipedia etc. that can guide you.

If you are just looking for a new source of entertainment because Hollywood has started to bore you or something, I'd advise against Bollywood. Take up knitting or something. Or maybe Korean movies. Something that has an easier learning curve, and better rewards at the end.