The maximum weight you can carry in Minecraft is complicated. For starters, ice is better than anything else because the max amount of blocks one water can make is ~537 million blocks^, and ice stacks to 64. Then you use shulkers and stuff. A cubic meter of water is ~1000 KG, so the max weight Steve can carry in two hands is...

45,778,176,000,000,000 kilograms (45 quadrillion kilograms)

• ^ Citation: Alexa real on YouTube*