Wow, there is a lot of latitude in that question. You list revolvers but don't say it is limited to just revolvers.

The most popular caliper could be the .38 S&W and it may be the most produced handgun caliber. Also popular with .357 mag owners.

Along the modern [last 50 years] revolvers the 686/681 & 585/581 series might have the most fans and received the most accolades. I could be wrong. That could well be my personal bias speaking.

Looking forward to reading more responses.