The 'best' antenna is
- any sturdy outdoor suitable antenna,
- tuned for the frequency of 1090MHz (not a broad band antenna),
- with not too much gain (3-6dB),
- located as high and as free of obstructions as possible,
- connected with good cable,
- as directly as possible (i.e. avoiding any adaptors).
The most important point of the above is probably "as high and as free of obstructions as possible".
A pre amp might not be necessary if the cable is of good quality and less than 10m length or so. A pre amp should be located right below the antenna, not at the end of the cable.
Give the supplied antenna a try, it works quite well. Put it in the loft or on a window sill, where ever it has a good 'view' of the area/direction you want to monitor.
Cheers,
Ekki/WiMo