Main thing for me is a wide range of leaves with as much taste as possible, so grown in season and in good soil with compost. Summer flavours are more delicate although colours can be strong, such as red lettuce, whereas now the range is broader with mustards, many other brassicas including rocket, chicories and endives of many kinds, chervil, winter purslane. lettuce, spinach etc!
In winter I love land cress in a salad with orange and parmesan crisps, so full of flavour and textures.
In summer it is much harder to choose as there is so much to choose from. In season home grown cucumber takes some beating. Also we put strawberries in our summer salads, they go surprisingly well with Cesar dressing.