If you are looking for good old hack n slash action, wait until you start running Nephalem Rifts which are designed exaclty for that purpose. Even though the number of enemy is not unlimited the density is really high and once you've complete a Nephalem Rift you can directly jump to another one and start slaying hundreds of mobs again.
If we are talking about infinite spawning mobs late in the game there is one location in act V that is pretty famous. Just follow those steps :
Note that this method has been nerfed, the monsters still spawn but no longer gives XP or loot.
- Go in campaign mode and Select : Act V > Quest 6 The Battlefields of eternity > Subquest "Siege Runes"
- TP to the Abandonned Siege Camp and run to The Battlefields of Eternity
- You need to destroy two runes. The first one is always the same (and doesn't need to be killed). The second one, however, is random. You are looking for an enemy named Emikdeva who is part of the event The Demonic Prisoner. If the second rune is not guarded by Emikdeva : quit and restart.
- When you find him, you activate the rune. Then pull Emikdeva as far away from the rune location as you can (withou killng him) and die. When you respawn go back to the rune (without aggroing Emikdeva back with you) and start killing the minions that spawns arround the rune.
- As long as Emikdeva doesn't die, they'll respawn indefinitely.
- Slay monsters.
Here's a short video I could find on youtube demonstrating this exact method :