I agree with Sue that testing your mother would be at the top of my list. The number of cousins and the quality of the predictions are significantly better when the relationship is closer.
Subscribing to multiple DNA services significantly increases the probability of finding a maternal cousin.
My Priority for any tester, but I am focusing on your mother, or any other maternal aunt/uncle.
1. 23andMe - The IN COMMON WITH feature will also identify those common matches that Share segments. You have to ask them to share this information which can be cumbersome at first. if these common cousins have a "yes" under the column shared, these 3 DNA testers have been triangulated. I think that 23andMe has reached 3 million subscribers. Also, it predicts your haplogroups, which may be helpful,
2. AncestryDNA - This is a very close second. I use Ancestry.com for my research and I find connections to more distant cousins this way. I find that most of these cousins have no idea about gedmatch or wikitree. If I wasn't on AncestryDNA, most of my distant cousins would not be on gedmatch or wikitree.
3. I would transfer my ancestryDNA test to FTDNA.com, myHeritage.com, and www.gematch.com (and its beta database Genisis). These are free, and it may also connect you someone from a Service that you do not subscribe to.
4. Subscribe to FTDNA yDNA (males) and mtDNA (females). Even though you are looking to extend your maternal line, I have not personally had much success matching anyone that was not matched using auDNA.
This is only my suggestion base on my own personal experience.