There is nothing that compares to the OED if you are serious about knowledge. Knowledge obviously depends heavily on language, but of course language is nebulously defined: The same word means different things to different people and in different places and times.

The OED uniquely solves this problem through an unbelievable amount of scholarship into each word's range of meanings, providing incredible breadth and depth, back to the known beginnings of its usage. If you are serious about knowing what something specifically means, it's essential. As one simple example, I find it to be the best source for really grokking mathematical terms by far, and also sometimes the meaning of the 19th century mathematician is different than the usage in 2018.