I've always loved chilaca chiles and I grab them whenever I find them fresh. They're long and skinny and rather thin skinned but you still want to roast and peel them before using them. When dried, they're called pasillas, and the dried form is equally delicious, just completely different.
Diana Kennedy, in her book My Mexico, has a good snack using the fresh chiles. After roasting them, you clean them and shred them with your hands. Toss the chiles with some raw, chopped onion, some cotija cheese and a bit of lime juice. It's makes a great taco filling but you could also serve as a garnish for any grilled steaks.
If you have trouble finding the chile, try shopping in stores that cater to expats of Michoacan and you'll probably have better luck. Kennedy suggests substituting poblanos, which you'll need cut with a knife rather than shred, and they're just as good, but different. With poblanos, I like to add some chopped epazote to give it a little edge.