USPS service in my area has always been reliable. There have been a few packages I've sent or received over the years that have gotten held up or lost for a few days, but nothing serious so far. I've gotten a few damaged packages, usually due to the sender's poor packaging, but everything has made it here intact.
Every couple months, I get notices in my mailbox about them looking to hire mail carriers. It's a rough gig, from what I understand.
As for UPS, when packages arrive, the boxes are more often than not in rough shape. I've had 2-3 instances where items were actually damaged enough where I had to contact the retailer about the issue. I don't sent out via UPS mainly because with my experience, they seem to be a lot more rough with packages than USPS or Fedex.
For Fedex, I've never had a problem. The boxes always arrive here in good shape, and I haven't gotten any complaints about the boxes I've shipped out. I've also received Fedex freight shipments a handful of times from both across the US and Europe. The delivery on one shipment needed to be rescheduled since it missed a connecting train somewhere, but otherwise, they were all delivered within the expected window safe & sound.
I've only ever had something delivered via DHL once, but that was probably over 10 years ago. Otherwise, I haven't encountered them and have no comment to offer.