if you want an easy dungeon campaign without much fluff, just go with the Greyhawk stuff in the core rules. But beware, it can be a little generic and under emphasizes the non-human races and cultures.
if you want literally every option ever produced for any game anywhere to be somewhere in the world, go with Forgotten Realms. But beware, the land is populated with people who are so powerful your players may feel secondary.
if you want a powerful sense of history giving the game huge context, go with Dragonlance. But beware, this is a land where huge events have already occurred and the great heroes have already existed. There is a great deal of historical weight to get past.
if you want gothic horror, go with Ravenloft. But beware, there are a number of alternate rules (horror and madness) that must be learned to play this one.
if you want a high powered tour of the endless multiverse, go with Planescape. But beware, just learning the ideas and terminology of this system took me a few weeks--its a very rich setting and thus potentially very complicated. Plus it is no long officially supported.
if you want magic suffused into every aspect of daily life, without demigods walking the land, go with Eberron. But beware, some players find it too "techy" and not "fantasy" enough for their tastes.
iif you want high powered magic/psionics in a harsh desert world, check out Dark Sun. But beware, this is a harsh world with many optional rules. Plus it is no longer officially supported.
if none of these sound good, look for one of the hundreds of other options or create your own.
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