Fryer's Delight used to be good, but in the last years has gone downhill.

Masters Superfish: I like them; had never had the problem JAY71 mentions (heavy batter), loves the ambiance (very simple interior, patrons mostly working persons - at night lots of cab drivers, no pretentions there), huge portions, both fish and chips always crispy. Just a curiosity: the owners (for the last three years at the least) are a a Chinese family, the cooks are occidental, don't know their nationality.

North Sea Fish Restaurant: a more upscale restaurant, with a take away at the side door, it is very good (a little bit better than Master's Super Fish, in my opinion). It is located between Russell Square and King's Cross/St. Pancras Tube Stations.

http://www.northseafishrestaurant.co.uk/