Romaine lettuce. If my memory serves me correctly, Romaine is the foundation for the well-respected Caesar salad, which is essentially the lettuce accompanied only by dressing and croutons. It is also superb for sandwiches and makes a lovely garnish.
Romaine's dark green color means a somewhat bitter yet sweet taste. When picking a head from your grocer, choose one with broad, loose leaves. Unless you are using the lettuce for garnish, it is advisable that you not include the tough ribs or veins of the leaves as they are not appreciated.
As with all leafy greens, brown means bad. If it looks bad, it probably is. There are two different varieties, the green (extra-crisp) and the red (lots of Vitamin C).
Romaine is fundamental to Greek and modern Italian cooking.