Hey
I'll throw in my vote for Waterdeep as well ... for the price of an ESD the City of Splendors boxed set will give you everything you need to run the city (yes, it's 2nd edition, but remarkably easy to convert).
The beauty of Waterdeep is two-fold for a group of hack n' slashers, as the deepest dungeon of them all lies beneath Waterdeep - Undermountain. Enough info is provided in the City of Splendors set for you to start mapping Undermountain yourself, or if you're not a DIY type, there is a ton of downloads that detail the upper levels. You literally cannot cover every inch of this place in a campaign - it's that big.
Combat-intensive scenarios can also be played out above ground too. A former campaign of mine involved a siege of the city by Amn, trade rival. Mass combat, sorties against the besiegers using portal magic, and attacks from Waterdeep's sky cavalry kept my PCs rolling d20s for an awful long time. Beyond a war scenario, Waterdeep lends itself very nicely to swashbuckling adventures, duels amongst the nobility, running from the city guard, bar-room brawls with foreigners in the Dock Ward, raiding crypts in the City of the Dead, and forays out of the city to Ardeep forest and the Mere of Dead Men.
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