The coastal cutthroat (Oncorhynchus clarki clarki) is our native subspecies of cutthroat. It ranges from the Eel River in northern California north to the eastern side of the Kenai Peninsula in Alaska and from the Pacific coast inland to the crests of the coastal mountain ranges. It is the only cutthroat subspecies that exhibits an anadromous form (sea-run cutthroat) but also has resident lake and stream-dwelling forms. Before the wide introduction of hatchery rainbows and non-native species like the Eastern Brook Trout they were the most common trout to be found in the Pacific Northwest. Since westlope cutthroat (O. c. lewisi) were never widely stocked in any but high elevation lakes on the western slope of the Cascades, the coastal cutthroat is still the one most most commonly found in most lowland lakes in western Washington.
Since they are still so very widespread in lakes, it would be hard to pinpoint one in particular. Oddly, one of the best coastal cutthroat lakes around may well be Lake Washington in terms of both numbers and size (a couple of years ago one that weighed nearly 15 pounds was caught there). Unfortunately, these trout feed mostly on longfin smelt and immature sockeye salmon in very deep water and are mostly caught with downriggers and herring.
I have a couple of favorite cutthroat lakes but will refrain from naming them (one of them is no longer accessible). Take a look at the lake reports on this website and you might get some ideas for places to start looking.
Attached is a photo of the 15 pound cutthroat that Preston mentions was caught in Lake Washington. I saved it from a short article in the Seattle Times. It was caught in August 2002 with a downrigger and spoon combo, weighed 14.9 pounds, and was 29.25 inches long.
Most westside Cascade lakes and ponds have cutts in them unless they're managed otherwise. Pick a blue spot on the map and give it a shot. The more remote the lake, the better your chances of hooking up.
Any given population's number and size will depend on how much food is available. Unlike the slightly alkaline waters of lakes east of the Cascades, the slightly acidic waters of westside lakes don't produce as much biomass. Hence most of the coastal cutts you're most likely to find will be in the 4-5" to 10-11" range. Yes, there will be the occasional larger fish (12-14" < 18-20"), but the larger they are, the rarer it is to catch one.
Regardless of size, I think coastal cutts are some of the prettiest fish native to the Pacific Northwest. Below is a pic of a sweet little 8-9"er I found about 10 years ago at a beaver pond up on the Hancock Tree Farm, east of Seattle.
Squalicum Lake a fly only lake East of Bellingham has a decent population of native coastal cutthroat.
If anyone is interested you can sometimes find the Lake Washington cutts up on the surface during the summer at dusk chasing longfin smelt. At other times they will move into the shallows chasing baitfish and crawdads. Both situations provide windows of opportunity for the fly angler.
I've caught a couple out at Lake Sawyer in Black Diamond. That is a good sized lake though and more poplular with high powered bespeckled bass boats and power boats with various kids in tow. They are beautiful fish indeed.