My 2 favorites:
The first is only really good when you can find Meyer lemons in season. They're sort of like a cross between a tangerine and a lemon - a little larger, usually, than a typical lemon, and not as tart.
Get a large serving bowl. Zest one Meyer lemon into the bowl, and then squeeze the juice of that lemon and one additional Meyer lemon. Add about 2 tablespoons of good olive oil, and a quarter cup of freshly grated Parmigiano Reggiano cheese. Then cook one pound of pasta (we prefer a flat linguine or ridged penne with this, but really any will do). Save about 3/4 of a cup of the pasta cooking water before draining the pasta, and then drain it and immediately drop it into the serving bowl with the lemon juice, oil and cheese. Toss well and serve right away. It's excellent with some fresh basil torn or thinly sliced over the top, and a little good quality salt.
The second is: cherry tomatoes (raw, simply rinsed), fresh mozzarella (the real kind, in the brine) and fresh basil leaves. If the tomatoes are small, leave them whole; if they're larger, cut them in half. Tear or cut the mozzarella to about the same size as the tomatoes (about the size of dice in a board game). Tear the basil leaves. Combine all those in a bowl with a drizzle of olive oil. You want the mixture to glisten with the oil, you don't want the oil to puddle in the bottom of the bowl - so use the oil sparingly. I usually let that sit at room temp for about an hour so all the flavors combine nicely. No measuring is needed - just a container of cherry or grape tomatoes, about half that amount of mozzarella cheese, and a big bunch of basil leaves. Then cook a pound of pasta. A nice small shape works well, like small shells, or bow-tie, or small penne. Drain it and combine it with the tomatoes/cheese/basil mixture. The hot pasta will melt the cheese just a little. You can then serve this at room temp, or chill it briefly in the fridge before serving. Also, sliced pitted Kalamata olives are a good addition to this, but not necessary. (not the canned black olives that are artificially ripened, but the real Greek Kalamata ones that are usually in a jar, in brine).