I used to think loot driven, isometric, ARPGs* peaked with Diablo 2 which was such a unique, monumental and influential game. It spawned a few good quality copycats in the following years, and a ton of its mechanics have seeped into different genres, but not until around 2012-13 did we see that next step, and it wasn't Diablo 3 who did it, but with the release of Path of Exile instead (which is a truer Diablo 2 successor). That's when the genre really reached new heights in my opinion.
This was also the time around when games like Torchlight, Marvel Heroes, Dungeon Siege 3, and even smaller stuff like Van Helsing and Victor Vran made their mark stretching up to the release of Grim Dawn not that long ago, all contributing to make the landscape richer in general. Diablo 3 ended up being a decent game after the expansion, but did not really live up to the promise of an always online, ongoing service with continuous content or a real evolution of D2's core tenets when it comes to build variety and expanding loot mechanics. These days it feels like Diablo 2 still has more concurrent players and pull than D3, and Path of Exile has done nothing but continue to grow, establishing itself as a constant top 10-20 fixture on Steam and Twitch. I'm looking forward to what GGG do next year. Maybe the true peak is yet to come.
*I feel like the category of ARPG I'm talking about here really needs those qualifiers for this specific conversation. Stuff like Borderlands et.al, while very similar, are more of an adaptation to a different core genre in my opinion (shooters in this example), while the games am I talking about have those clear identifiers that connects them all together in a much more essential way.