I am using an Antenx All Band Scanner antenna and it works out good for me. It is an NMO mount and I am using a trunk mount. You can see a picture of it in the link to the thread below. Post #9 shows it on my car, it's the short antenna on the trunk. Hope this helps out.
Find out how strong the signals are that you want to monitor first,as well as if there is much interference nearby.
I kept trying better (higher gain,or band specific) antennas to try and improve reception.If anything,it kept getting worse.It was overloading the receiver,and causing problems.Now I use a unity gain all band low band antenna,which does well on everything.
You will have to experiment first,as it may not be the same in your location.If you find your 800 signals are very strong,but need help pulling in the lower bands,then a 2M\440 antenna will probably work well.If it's the opposite,an 800 Mhz antenna may work well.