The Maddison Project hosts a database of world GDP data. Angus Maddison was basically the most famous econometric historian. If you want international historical economic data he's your first resort.

According to this dataset the answers are:

1500 - Northern Italy, followed by Belgium and then very closely Holland/Netherlands

1600 - Holland/Netherlands, way out in the lead.

1700 - Holland/Netherlands, losing some of its lead but still clearly ahead. England/GB/UK is second.

1800 - Holland/Netherlands once more, England/GB/UK still second.

Of course there are plenty of problems with these figures, not least what counts as a "country". Estimates are completely missing for most European countries prior to the 1800s. I'm sure estimates exist, but I guess they couldn't find any that were both reliable and comparable enough to include. Germany, Spain and Portugal are included, but France isn't. Not that that would change the results above. Even for the best administered country, GDP estimates before the 19th century are very rough.

In answer to your second question, the fact that Holland/Netherlands is so consistently in the lead largely renders it moot. I would imagine that Northern Italy was eclipsed largely as a result of new trade routes and the nascent Dutch Empire, but I couldn't give an authoritative opinion. (Edit: just to expand on this slightly, Venice controlled much of the spice trade, until routes around the capes were opened. These both made other powers wealthy and directly hit in the income of the Venetians.) I'll give some of the basic reasons why Holland/Netherlands was so comparatively wealthy. I'm sure there are plenty more that could be added.

• Small, urbanised, fairly homogeneous, without a large, poor, agricultural peasant population or an undeveloped hinterland.

• Relatively fertile.

• Good ports and rivers for trade, both at sea and into the continent.

• Development of a trading empire, particularly control of the incredibly valuable spice trade.