Nobody can predict which database is going to have your best matches because it is somewhat a matter of chance of who tests where. Not all of your relatives or potential relatives are going to test at the same company. Also, the algorithms (mathematical formula) is not the same for each testing company.
The big 3 companies at the moment are Ancestry.com, 23andMe, and FTDNA. In Europe, it is GPS and Living DNA. You can test with the later if you have European roots.
I would highly recommend uploading your data to the free site www.gedmatch.com which acts as a central site for intercompany compares. Since it is on a volunteer basis, it only has some of the results from each company, but it does tend to attract the most interested researchers, and therefore should have a good response rate if you attempt contact. You can upgrade to Tier 1 for advanced data manipulations for $10.00 per month.
Once you get uploaded, try the one-to-many lookup. You can see the original source of a contributor by the first letter of his kit code. (A=ancestry, M=23andme, T=FTDNA).