My favorite Zoid is the Orudios. Originally introduced as an OJR, it was re-released under the Zoids Graphics line. The colors are patriotic, the chrome is awesome, and hey, I love horses. (particularly race horses-and the orudios is very fast!)
Before I knew about Zoids that much, I didn't know that the Orudios even existed. The only zoids released were the "kiddy" ones with ugly colors (I'm talking about Hasbro releases) and the old, cool zoids weren't really talked about.
Oh, and, did I mention the Gil Vader? He's my other favorite zoid! Gil Vaders are so awesome, and by far the coolest dragon toy--ever!
