I think Batavulus and Joonas are close, but I think the answers so far are hindered in attempting a literal translation. In your English sentence, joy and pleasure are essentially synonymous. You are in essence defining joy as pleasure, but which makes it redundant. Unless you want to specify what kind of pleasure, you're actually merely saying, "The joy you get from understanding is the greatest joy," or, "The joy you get from understanding is the best," or even "Understanding makes for the greatest joy." You can substitute pleasure in there with no loss, though the difference between joy and pleasure is enough for you to specify which.
Alternatively, you can use both in this way: "Understanding leads to the greatest joy and pleasure."
Technically, you could mean something like, "That feeling you get from understanding something (joy) leads to pure pleasure," but it's a nonsensical term. Are you aroused by happiness? Skip one and go for the other.
With that, I would give you the pithy: Intellegere est summum gaudium. / To understand is the greatest joy.
If you wanted to emphasize the pleasure aspect, you'd instead go with: Intellegere est summa voluptas. / To understand is the greatest pleasure.
For comparanda, cf. errare est humanum (To err is human) and the frequent use of summum bonum in authors like Cicero or Seneca (with a citation as early as Terence).
The former is the feeling of elation you get from understanding; the latter is the sensation of pleasure, which will be more sensual than the former.