Generally, the smaller the fish the less mercury. That's because big fish live longer and thus have more time to accumulate mercury.
Sardines, mackerel, herring, anchovies, etc, are very small and have little mercury. These fish also reproduce very quickly so they rarely become overfished and can recover quickly when they are. These fish are among the best sources of omega-3.
If you want tuna, stick to skipjack (usually sold as "light" tuna), which is one of the smallest (75 pounds) and shortest lived of the tunas (<4 years) so it has the least amount of mercury. It also reproduces quickly and is the most sustainable. Albacore are larger (up to 88 pounds) and much longer lived (7 years), so they will contain more mercury than skipjack. They're also slower to reproduce; most albacore stocks are doing alright, but it wouldn't take much to begin overfishing them. Bluefin is the worst, weighing in at an amazing 1400 pounds, and with a life span of over 35 years and a very slow reproductive rate, most fisheries are badly depleted.