Ko Ratanakosin, the artificial island from which Bangkok sprung, is one of the more atmospheric parts of town. Unfortunately, dominated by Buddhist temples and royal palaces, the area has relatively few restaurants. If you need culinary incentive to visit, there's always Pa Aew, a longstanding street stall near Wat Pho.
The older couple here do exactly the kind of rich, oily, spicy central Thai food I love. Just about everything here's fried, but fried with care. Unsurprisingly, the dishes are somewhat oily, but oily in a rich way and not necessarily greasy.
There's almost always kung thawt krathiam, large shrimp deep-fried with garlic (shown in the centre of the pic above), and quite a few other seafood-based dishes. On my most recent visit I had phat phrik khing, a thick curry-like stir-fry with a spicy/sweet chili paste, fish and long beans, and phat chaa look chin plaa, a stir fry of herbs and fish dumplings:
Both were rich, spicy, oily and tasty. Highly recommended.
Pa Aew near cnr of Th Maha Rat & Th Pratu Nok Yung (in front of Krung Thai Bank), Bangkok 9am-6pm
View Thai Eats in a larger map