"good" is relative two different creatures.
AM is usually satisfied by a plain old long wire, stretched out as long as it can go.
FM is a different creature. A lot depends on how far you are from the stations, in what direction it is (or they are). But, be aware that those tiny, indoor plastic miracle promising powered antennas disappoint ...big time.
If you are close, a simple $4.00 folded dipole, available at at RatShack. Lowes, Home Depot, Wal-Mart will suffice.
If you need more distance you can look at these, assuming you can mount 'em outside.
http://www.antennacraft.net/FM.htm
I use the FMSS omnidirectional since I'm surrounded by statiuons and and am more than satisfied. The FM 6 is directional and will pick up further stations, but only in one direction.
If you need evenmore distance, they are available. just ask. Search on "APS antenna"