Look for small leaf salads or bags containing spinach and rocket if you are after a more robust flavour. These leaves are packed in a mix of gases that keep them fresh for longer so once the seal is broken they will deteriorate fast. My advice is to buy, open and eat.
For whole lettuces, the variety you use is down to personal choice, but I like Batavia for general salad eating, Cos for a Caesar or crunchy salad and Little gem for something with a stronger flavour. Whatever lettuce you like, make sure it is the freshest โ look at the back of the racks as they stock back to front.