It is very difficult to make good idlis. It is very difficult to make bad dosas.
While it is difficult to make bad dosas, making a really good dosa is not all that easy.
A lot of things are important. The composition of the batter, its consistency, the time for which it is allowed to stand, the type of 'tava' used etc. etc. etc.
Coming to the composition, everyone says their's is right. It is mainly a mix of rice and pulses. The pulse used and the proportion again varies from person to person. Some people add 'poha' or fluffed rice and claim this makes the dosas light and airy.
The right consistency is very important. Too thick and it becomes like an 'uttapam'. Too thin and it is difficult to cook.
If the dosa batter is not allowed to 'rise', the dosas become more like leather instead of being light and fluffy (ever wondered how a thing so thin can actually be fluffy?).
There are many variants of the dosa. Plain dosa, which is the pure and simple vanilla dosa. Masala dosa which is a dosa with a potato/raw banana dry curry, Mysore sada and Mysore masala which have a red chutney that is smeared on the dosa and so on.
Pretenders have recently started listing "different types of dosas". A little investigation reveals, however that all they are doing is to use the humble plain dosa and top it with different toppings. Cheese dosa = Plain dosa + cheese, Palak dosa = Plain dosa + spinach. Please. These are not pizzas, for heaven's sake!
There are a few places that make really good dosas. In Chennai, Murugan Idli Shop's ghee roast dosas with about a kilogram of ghee per dosa are very good. The dosas at Woodlands and Sukh Sagar in Mumbai are also fairly good.
The prize, however goes to 'Dasa', a restaurant that had branches in Chennai and Hyderabad. The dosas there were truly heavenly. They had just the correct texture, color and taste. The butter they used made the dosa really tasty. They were never too heavy. Light, fluffy and truly a treat for the tongue. You could easily polish off three in one sitting and still ask for more. Your stomach would say 'Stop' but your tongue would say 'More'!
Unfortunately, really unfortunately, Dasa has closed down. Oh! what a loss that was, my countrymen!