That depends on your definition of "empire".
If you mean a colonial-commercial empire with viceroys and territorial governors, the answer is Spain from 1580s to 1640s.
If the answer includes a colonial empire with military power and European client states, the answer is Spain from 1530s to the 1590s or France from 1802 to 1814 and 1840s to 1870.
If you mean a military power with commercial influence over client nations, the US is the largest.
If you follow the ancient definition whereby any nation-state around a capital city is technically an empire, Russia is the largest.
The UK did have a large colonial empire, but it was nothing in size compared to that of the Spain-Portugal Empire from of the late 16th CE to early 17th CE.