For me, it has easily been the Gruv Gear V-Cart Solo
. I used many foldable carts in the past, and this one by far is the best. Easy to set up, many different configurations, very stable (as in, you don't worry about it tipping over with your amp when you're turning a corner, or going up and over a curb or any type of slant) and rock solid. Many people shy at or poo-poo the price, but I have honestly not found a cart that compare at all.
I was saying this all prior to my endorsement with them, and it hasn't stopped. It's a great piece of gear.