The Worst ever, was a panasonic 8.4" HD, BTLH900a that cost me $5,000 and was finally sold at a loss of $3,000. Got only one $150 rental on it once, thats it.
Depending on how you work there are some investments that are still worth it just to make your life easier. I just bought some Sachtler Speed Lock CF legs because I don't like bending down to adjust the height of the tripod. I can't charge clients anymore for them but I only wish I'd bought them years ago. I guess a rental firm may be able to charge more for such things but as an owner/operator it's hard to justify an increase in day rate.