Remember that antennas are frequency (or more generally band) dependent, not radio or transmission type (digital, analog, etc. are the same to an antenna, but VHF-Low, VHF-Hi, UHF, and 800 MHz bands are different to an antenna). A great antenna for 800 MHz will probably work well on 800 MHz systems on both a Uniden and a GRE scanner, but may work very poorly on either scanner when trying to pick up a signal on 30 MHz.
If you can let us know what band(s) you're mainly interested (in priority order if you have more than one band) you'll probably get some good recommendations. Without this information we can only guess and our favorite may work well for the band we listen to, but work very poorly in your favourite band.