What are your recommendations for a virtual assistant service (alternative to Zirtual)?

I previously used Zirtual for a few months, and really loved it (but my cash flow was very limited so I had to cancel it). The company has since imploded, so I'm hesitant to hire them again. Do you have any recommendations for alternatives? I am looking for US-based, and higher quality (not just someone to book meetings but also someone who can do research, create spreadsheets, create letters based on a template but that need to be customized, etc). Any recommendations?