Hello Andrew,
This is a difficult question. It's like asking someone to choose only one of hundreds of types of cheesecakes, brownies and ice cream.
I will give 3 plausible answers:
- Brightlocal.com
Brightlocal is my go-to answer for all things local SEO. It gives so many benefits that I don't think I could live without it on a local scale. Benefits include:
- Rank checker (SERP Rankings)
- Website Check-up (site auditor)
- Google+ Local Wizard (Checks your Google+ account and can be used for competitor analysis)
- Citation Tracker (Small link-building potential)
And there is still more coming.
The beauty of this tool is not only in its accessibility and general usefulness but also that it is an all-in-one answer to most of your local SEO needs. This means that with 1 tool even a start-up business could compete with established businesses and build their client base. This makes it extremely powerful, especially in the scenario you set up.
- MajesticSEO or Ahrefs
These two products effectively do the same thing - competitor analysis, site metrics and ranking. They are both very powerful and useful to have. I personally prefer Ahrefs because I have used it more than MajesticSEO, but both have a good product. I typically use one or the other as part of a greater strategy, so it is hard to say that these would be the only tools I would want in my bag, but it is a great starting point from which you could work if it was all you had.
- SEO Powersuite
I don't know if this is cheating based on the scenario you set up, but I am going to go with it anyway. The SEO Powersuite is what I would want up my sleeve for any SEO challenge. It encompasses most technical aspects of SEO within 4 tools: SEO Spyglass, Rank Tracker, Website Auditor and Link Assistant. These are all powerful tools in their own rights that allow you to formulate raw data, measure and analyse it, and to create professional customized reports for clients. It is technically 4 tools in 1, and can be pretty expensive for a start-up business, but it is definitely worth it if you can afford it. I would take the Powersuite over even Brightlocal, but I have a feeling that this wasn't in the spirit of the question you were asking.
Can't wait to see what others have to say on the topic!