Can it be a custom potion as an item? I'd go max time regen/jump/speed potion if it were allowed. Otherwise, definitely an efficiency 5 diamond pick. If it can't be enchanted then I'll just take a debug stick.
Why a gold block when you could get a diamond block? Diamond is worth so much more than gold
Any value diamond has is from being artificially marked up by the diamond industry. Diamonds are extremely common and can be easily manufactured. Not to mention they're mostly useless. Gold on the other hand is actually rare and has some real useful properties which makes it much easier to sell. Gold also can't be manufactured. (Unless you count making a couple hundred atoms for $100K that are also probably radioactive.)