Well two powerful matriarchs that I wouldn't like to tangle with are Galadriel and Lobelia.
Lobelia is probably the most traditional type of matriarch and a character type I instantly recognise: the formidable wife and mother who is quite determined that her family, her husabnd and her son will do well for themselves, and what's more, be seen to be doing well for themselves. Anyone in the UK will know what I mean if I raise the name of Hyacinth Bucket (pronounced Bouquet).
But for all her snobbery and pushiness, Lobelia has a good heart in the end, and manages to battle the ruffians with the aid of her trusty umbrella.
Galadriel appears to defer to Celeborn, but we know who is in charge. She first has the Elessar and later has Nenya, both crafted by Celebrimbor and gifted to her (he is in love with Galadriel but his love remains unrequited, which hints that she exerts a powerful romantic attraction too), and she makes use of them to strengthen (to create?) the magic of Lothlorien, a place which seems to act as a 'temporal shift'. Galadriel seeks to exercise power, certainly at first, and she's just one example demonstrating to us that Tolkien was not (as many hapless critics say) dismissive of women. I wonder what her apparent public deference to Celeborn tells us about Elven society?