The factory one really isn’t bad at all. If you’re dissatisfied with it, then it would help to know why and on which frequencies you feel it’s underperforming and in what kind of environment(s) you’ll be using it. Will it live on your desk, in you car, or ride on a belt clip as you wander the state fairgrounds, or what?
I’ve found the Watson WSMA-801 to be a pretty good option for something short and flexible, and the Diamond SRH77CA even better for VHF reception—though at ~16” long it’s more unwieldy. I use the BNC version (RH77CA) together with the excellent Diamond BNCJ-SMAP adapter, because there are just so many good BNC options out there, and the adapter sits tight against the scanner body and seems like it would help spread out any shock load rather than relying on the native SMA antenna connector itself. If you’re even slightly clumsy it’s a worthwhile thought.
Nothings going to be an external antenna mounted properly on your vehicle roof or building roof, though.