There's an amazing variety of dungeon board artwork available nowadays.
We wanted to dungeon dive, but not spend a ton of time and money making full 3D terrain.
The compromise: make 2 and a half - D terrain, cutting out floors and walls and mounting them on Foam Core to represent different levels.

We're really happy with the Heroic Maps Great Dungeon tilesets, since you can lay out lots of different variants very quickly.  It took about two months to make 2.5D versions of all these tiles...

We even made some tiles with cutouts in the floor, allowing us to put different backdrops below for a variety of effects.

They even have a nice Cave Tiles set (although the connection points are a different size).

Our other favorite source for dungeon board art is Seafoot Games.  These large boards are not modular, but look great.  It's a bit more work to make 2.5D versions of these boards, but thankfully most of them fit well on the 20x30 inch foamcore you can find at Dollar Tree stores.  (You just have to paint the edges of the foam core black for best results).

This 2 level crypt was one of the first boards we played on in our campaign.

We have yet to play on this undersea board, but looking forward to it later in the campaign.  

This massive coastal town required FOUR 20x30 foamcore boards and barely fits on our table.   We use the boards for our town of Scrimshaw.