So quickselect needs multiple passes, and the heap needs O(n log k) time to find the top k elements of n elements total.

However, you can find the top k elements in O(n) time and O(k) space in a single pass.

One simple way: you keep a buffer of up to 2*k elements. You scan your stream of n items one by one. Whenever your buffer gets full, you pare it back down to k elements with your favourite selection algorithm (like quickselect).

As a minor optimisation, you can only add items to your buffer, if they improve on the worst element in your buffer (or when you haven't hit k elements in your buffer, yet).

As an empirical question, you can also experiment with the size of the buffer. Theoretically any multiple of k will do (even 1.1*k or so), but in practice they give you different constant factors for space and time.