Right now, I'm eating a lot of wilted kale salads with cooked beef on top, while waiting for the weather to finish warming up enough for summer salad season to start.
Romaine's a mainstay for me; I love the crunch. It also lasts longer than other lettuces; I can chop up a week's worth of Romaine for Big-Ass Salads, put each salad's worth of lettuce in a gallon-size Ziploc bag, and the lettuce for the last salad of the week is still good by the time I use it five or six days later.
Last year, I grew butter, oak leaf, and a really deep maroon red-leaf lettuce in boxes on my deck, and they were great. I also planted miner's lettuce, which died before I could enjoy much of it--but now it's self-sown and grows rampant all over my yard. I can just go out and cut a bunch whenever I want it. It has a mild flavor.
I also grew herbs last year, and chopped up lots of tarragon, basil, cilantro, dill, and/or Italian parsley straight into my salads, rather than just using a little bit in the dressing. Another one I like a lot is nasturtium; the leaves and the flowers have a peppery flavor to them.
And if your guts can handle raw cabbage, it makes a really tasty salad, too--especially if you let the cabbage marinate in the dressing for a little while before adding the other ingredients.
Mmmmm, salaaaddd...I know what I'm having for dinner tonight...