It boils down to what kind of timeline you hope/expect to have before needing to draw from that balance. Some people plan to grow their HSA balance all the way until retirement. Others use it more as a supplemental emergency fund (potential to draw earlier). Others have enough ongoing medical expenses that they keep their HSA primarily cash to cover those, getting only the "tax free on contribution / withdrawal" savings, skipping out on the "tax free growth".

The longer you intend to leave the funds invested, the more likely you are to ride out any downswings of more volatile (though often higher return) investments like stock funds. Over the long term, things like stock funds are somewhat expected to grow more than a conservative option like a CD.

So the answer is "whatever fits best with your financial situation and plans".