I always have to make a korma for the girlfriend and find it works really well to show variety. The difference in colours (as well as taste obviously) is impressive for balance. Even my blokey/macho friends agree that it's a goodun. The fact it's so easy to cook and can be cooked at the same time as others in a small saucepan make it a firm favourite.
Either a dhansak or patia are now a must due to the mango chutney and ketchup used respectively. As well as chilli and lemon juice of course. Whenever i have used chutney or ketchup the curries have been enormously popular and of BIR standard.
I suppose my only other 'must have' is the madras.
I get worried that dishes like rogan josh, dupiaza, balti, karai, jalfrezi etc are too similar. What i'll do sometimes is use a different curry powder when making two similar curries. Jeera chicken's a good one as it only involves frying some whole cumin in the oil but gives an impressive result.