Back to the original post/question. I don't think that there is a "best cover". Covers range in price from about $30 (1/2 cover) to $250 full trike cover. None of them will keep a trike completely dry or protect it completely from the elements. All will discourage people (kids and drunks) from crawling on your trike and playing with the controls.
Covers range in price based more on the perception of quality and brand name recognition more than on the utility of the product. I use inexpensive, light-weight (compact) 1/2 covers in the $30 to $40 price range.