Ok, I will try as I am not sure the OP means a generic dish, a dish made at home, or a dish made at a particular restaurant. As with everything, many dishes can be great if well made, especially in fine dining restaurants where what the chefs are making that season may be great or disappointing.
Indian food
Kheema (minced meat),
Any fish curry or fried fish well made in Mumbai or Goa,
Some of the authentic breakfasts and desserts not available outside India,
Only place I like Indian food outside India is Dubai, and briefly tried in Qatar, was delicious.
Sri Lankan food
Best way to get Indian food in London. Sri Lankan food is awesome, and spicy.
The mutton curry with string hoppers at Everest Curry House in Lewisham, or the paratha, mutton, and egg all wrapped up in a banana leaf at Palm Beach at Wembley Central. Hoppers is decent but not like the other two, which are more authentic.
Biryani
Best Biryani I have had is at Darjeeling Express in London. At their special Sunday Biryani lunch, a 3 hour 7 course meal. If you are visiting London, make sure you book a lunch here. The restaurant is not good for their day to day food.
Italian
I love good Italian, and my favorite Italian in London shut down during the pandemic. The others like Locanda Locatelli, Padella, Cafe Murano, Cecooni, and others are ok, Daphne and Zafferano are disappointing. In fact the bets pasta is now usually when a fine dining restaurant like Kitchen W8 or Club Gascon makes a Gnocchi or Risotto. But otherwise, always love trips to Italy. Freshly done pastas with good sauces are great.
Sushi
Best Sushi at Yashin, off High Street Ken, their platter and sake is great, and then Umu.
Breakfast
To make at home, Turkish eggs (we have an awesome recipe), and my own masala oats for staple, which I sometimes vary as oats omelette, or oats pancake, with spicy chutneys.
Otherwise, Wolesley for their Haggies, Sauteed Liver, and Kedgeree.
Fine dining
Lyle's
Kitchen W8
Love good dishes made by chefs with surprise, can be small, where the flavors just build on each other.
Chinese
A. Wong at Pimlico, and there is a small restaurant at Angel can't recall the name. Great Cumin beef.
Thai
Herron at Paddington in London serves Northern Thai and they have a great pork and rice dish
Addie's Thai at Earl's Court for the Deep fried cod with morning glory and their Soft Shell Crab
Desserts
Various types of Pavlova Eton mess type messes, Tiramisu (but difficult to find a good one, again went with that Italian restaurant that shut down in the Pandemic), Rabri (a condensed milk sweet) that you get at many places in Dubai, and India, but not a good one in London unfortunately. Mango or Pineapple sheera (an Indian sweet made with ghee and wheat flour and sugar adding the desired fruit). At home make tasty healthy sweets mixing yogurt with tahini and fig etc.
Unhealthy snacks
Good samosas and vada pav (Indian)
Ok tired writing now.