A Reuters article, Trump's tax cut won't be the biggest in U.S. history looked at this claim in early November, 2017.
They do consider various definitions that might be used.
If it were measured only by percentage points off the top corporate rate, it would be the largest:
The president and his fellow Republicans in Congress propose cutting the top corporate tax rate to 20 percent from 35 percent. If they succeed, it would be the largest American corporate tax cut since the modern corporate tax began more than a century ago.
As proposed by the Republicans, the corporate rate would fall 43 percent, compared with the second-largest such cut of 26 percent under Republican President Ronald Reagan in the 1980s.
However, they dismiss this because it isn't the claim:
Trump would be on target if he were talking only about corporate taxes, but he has included other taxes in his boast.
If it were measured only by percentage points off the top individual rate, it would not be the largest:
Some Republicans in Congress are backing away from cutting the top individual tax rate, now 39.6 percent. But even if they did cut it as low as Trump wants, to 35 percent, the president would not even come close to winning bragging rights.
[...] the huge tax cuts of President Warren Harding and President Calvin Coolidge, both Republicans, take the prize. In 1922, the top tax rate was 73 percent. By 1925, it was only 25 percent, almost a 66 percent decline. Coolidge alone was responsible for a 57 percent cut in taxes.
If it were measured only by the total tax takings, the question becomes rather murky, because the takings depend on the economic activity, and the taxes themselves will likely affect that - to what degree is disputed.
However, they have one estimate that suggests it won't be the biggest ever:
The Committee for a Responsible Federal Budget, a bipartisan Washington think tank, estimated the Trump tax cut could be the fourth-largest as a percentage of gross domestic product, or GDP, a measure of national economic output.
A link to that committee's blog [Hat-tip to @tim for the link.] shows that Reuters appear to have made an error in citing that statistic.
CFRB estimated that it would be the eighth-largest as a percentage of GDP. It would be fourth-largest in inflation-adjusted dollars.