does nobody else not have a least favourite?
Hmm well if we're being honest we all have a game we prefer and like the least. I do think all the games have merit although from the main line games Dual Destinies does ruffle my feathers the most. It's too easy, the dialogue/translation is rather weak and are more childish than the previous games, the characters they added are pretty forgettable, and the investigations are rather pointless due to no free will. The pros are that the focal plotline is very good, the DLC case is well written, and the character specific themes are incredible.
...It's just that it strips aways so much from the series mechanically. The free will investigations, the lawyer powers actually have difficulty, and so on are needs for the series.
In terms of story I find the first Investigations to be the worse. The Yatagarasu was interesting, but it took half the game to get going and I didn't feel the impact of its conclusion. The smuggling ring was weak and the finale drags on for no reason. Also case 3 is really bad which doesn't help matters. Case 4 is a gem though.