My 'never ever again' is San Angel Inn. We've tried it 3 times and won't be going back.
On the other hand, the only place I must go every trip is Mama Melrose. I love how accommodating they are there. I usually get the Spaghetti Fra Diavolo, but always get it with shrimp only and with the non-spicy marinara. Their chocolate cake is one of my favorite desserts.
Of the two new places at AK (Tusker House and Yak & Yeti), we liked Tusker House okay. Y&Y just wasn't very good, but I do think our opinion was shaded by the one of worst servers we've ever had. I had the steak and shrimp and while the shrimp wasn't bad, the steak was terrible.
There are two places we'll only go a certain times. LTT is great for lunch, but we don't care for dinner there and Rose & Crown is great for dinner, but not for lunch.