Due to type erasure, the Stream implementation has no knowledge about the type of its elements and can’t provide you with neither, a simplified max operation nor a conversion to IntStream method.
In both cases it requires a function, a Comparator or a ToIntFunction, respectively, to perform the operation using the unknown reference type of the Stream’s elements.
The simplest form for the operation you want to perform is
return countMap.values().stream().max(Comparator.naturalOrder()).get();
given the fact that the natural order comparator is implemented as a singleton. So it’s the only comparator which offers the chance of being recognized by the Stream implementation if there is any optimization regarding Comparable elements. If there’s no such optimization, it will still be the variant with the lowest memory footprint due to its singleton nature.
If you insist on doing a conversion of the Stream to an IntStream there is no way around providing a ToIntFunction and there is no predefined singleton for a Number::intValue kind of function, so using Integer::intValue is already the best choice. You could write i->i instead, which is shorter but just hiding the unboxing operation then.