Romaine the "Caesar salad lettuce is dark green, very firm and crunchy. It keeps very well, and its tough, tight heads ship and transport without damage.
Green and red leaf lettuce varieties are sweet and tender. Because their heads can be quite large, you'll get great value for your money.
Frisee, curly endive and tango lettuces are frilly and very chewy greens that add crunch to a salad. They tend to work better in tandem with tender greens, creating a variety of textures. Chop them in smaller pieces.
Arugula and watercress add a spicy kick to tasty side salads. Their strong bite is usually too much for a main course salad.