Fish!

The main concern with fish is mercury. My main protein/meat source is fish as well. It mainly depends on type/size of fish. Large predatory fish like shark, swordfish, king mackerel, and tilefish all typically have high levels of mercury. I would eat these only occasionally

Salmon, anchovies, herring, sardines, trout, and Atlantic and pacific mackerel on the other hand, are low in mercury and high in omega 3 fatty acids (which is good).
I personally buy a lot of salmon and farm raised tilapia. It's relatively inexpensive.

Here's a chart with different mercury levels.

Anything I've read up until today about mercury levels are for the waters around North America. The absolute worst was Swordfish from the Gulf of Mexico. Basically, any fish that mucks around on the ocean floor or eats other fish (higher up on the food chain) will have higher levels of mercury.

BUT you have to eat a freak load of fish every week to get anywhere near dangerous levels.

Here's a study from the Indian Ocean.

"The muscular Hg levels presented here suggest that consumers of fish originating from the Western Indian Ocean should limit themselves to one Swordfish based meal per week, or one fish meal a day if they choose to eat tuna or Common Dolphinfish." http://www.sciencedirect.com/science/article/pii/S0048969706001124