When you've carefully sourced extremely fresh oysters, a really great way to actually taste them is to keep the condiments as simple as possible. That said, adding condiments is a great way to play with flavor. Since salt and acid foods balance each other, you can't go wrong with some lemon.
Whatever you do, remember that all you need is a few drops.
A classic mignonette is amazingly simple: red wine vinegar, shallots, and cracked black pepper. From there, you can get as creative as you like, switching up the vinegars, adding white pepper, adding jalapenos and cilantro and lime (as in Hog Island Oyster Company's "hogwash").
Another classic European way to serve oysters is with bread (sometimes brown, but always thick and chewy), sea salt, and farmstead butter. Most of these European oyster traditions were created for the European Flat or Belon oyster, and that oyster is truly a mouthful. It cries out for something to wash it down. Our pacific oysters are much friendlier, and easy to overpower with too much condiment... but it's still fun to pretend you're in Paris.
We love freshly grated horseradish root on oysters, and almost always serve it at our "serious" oyster tastings (as opposed to those times we're just leaned over the kitchen sink). The grassy bite of fresh horseradish is an amazingly complement to the fruity saltiness of a fresh raw oyster.
Heat is a great way to spice up an oyster, and it's such a classic American way to serve them that it never really gets old. We really love Fred & Floyd's Hot Lime Sauces, because they complement the oyster a bit more than a red hot sauce typically does. And remember to take it easy... some hot sauces pour pretty fast!