I went away for a couple days and left my plants on my porch. It was rainy and humid for the whole weekend and then some, causing a few of my plants to get rot. ):

I did manage to save this guy I think. I felt the base and it was kind of squishy, so I pulled it out of the pot and very moist mix and set him out to dry. It's been about a day now, and there's no improvement in the squishyness. However his roots look pretty good, and the base doesn't look sick really- just one brown spot.

Should I be concerned about the brown spot? Should I cut it out? I'm really worried about this one and any tips would be greatly appreciated!

I would just continue to let that cacti dry out more. It will hopefully scab over. Try to wash off the wet soil for now and let it dry for a few more days. It can take it. It has enough moisture in its body. You can also dab some cinnamon on the brown spot if you want.

Yes, too much rain is bad for most cacti, unless you have planted it in several layers of very porous or rocky media so that the base does not sit on wet media and water will just quickly pass through.

Once it has formed its scab, plant it again this time in a more porous, grittier media.

Thank you so much! I will try that and let you know how he's doing in a few days.