Howdy - Echo the above post....I think the first question set would be around what are the Bands that you are likely going to scan the most?
Thus, if you have just wanted to scan Civ Air or VHF High Band, then a more dedicated approach to the antenna selection might be better. If you are scanning the world Low VHF to UHF then a Discone is likely your best bet....
Getting the antenna high is important, but a good quality COAX and good quality connectors is really important and often over looked...have seen guys have a great scanner (SDS-100) and a great antenna and the cheapest coax they could find and then complain that the antenna or scanner is lousy. Invest in good Coax (LMR 400 as an example) and a good Discone and that's the ground work to better scanning in my opinion.
I took a different approach and use multiple antenna's for selected Bands.
Example for UHF Mil-Air I use a AT-197A/GR and for 700/800 Mhz I use a Larson type antenna fit to frequency band antenna. For Ham / VHF High Band I simply use my Ham Radio "dual band GP-3 type antenna with a Coax Switch to hone in on a specific band - when scanning all bands I can use the GP-3 as its about 24' up on a mast and works well enough for my needs...if you are in the sticks and want to pick up say a UHF 800 PD/FIRE P25 band you will likely need that YAGI, as noted above, that points directly to the transmitter site to get the receive gain & that's still no sure bet if its a log way away.
Every station and operator have difference needs and situations, so this is just one of a million examples..