I would first go to
http://www.antennaweb.org and see how far you are from the hdtv transmitters and what direction the signal is coming from.
You can pick up a real cheap compass in the camping section of Walmart.
I'd probably also recommend getting a cheap 25 - 50 ft. length of tv coax cable from
Monoprice.com so you can move the antenna around and see where you get best reception.
You have to figure out if your main problem is multi-path, or just a weak signal being transmitted from the tv stations.
Zenith / Phillips Silver Sensor HDTV antenna is "the" hdtv antenna and is supposed to be very good at multi-path rejection.
I've also got a Terrestrial Digital DB-2, which is supposed to have higher gain, but I still tend to prefer the Silver Sensor:
Link
My personal observation is that the tuner chip in your HDTV is very important (I'd recommend a 5th generation tuner, at least, as these are supposed to have very improved multi-path rejection).
YMMV.