There's a lot, from a variety of genres. It's also subjective. To make it easy we could just say prominent or popular and that would make it much easier. Rush, Joni Mitchell, Shania Twain, Bieber, Drake, The weeknd, Hank Snow, Oscar Peterson, Neil Young and Celine Dion are all music industry giants and all Canadian but they mostly wouldn't be my favorites. So, are they the "greatest" because they are huge? Not really. I defy you to find much of anyone on this forum (since we're not 12 year old girls) who defines Bieber as one of the great musicians of our time.
Canada punches above its weight music-wise though. Lots of great bands and solo artists.
Take your pick from Tragically Hip, Rush, Bryan Adams, The Band, The Guess Who, BTO, Alessia Cara, Shania, Celine, Triumph, Tom Cochrane, Gordon Lightfoot, Joni Mitchell, Neil Young, Leonard Cohen, Oscar Peterson, Drake, The Weeknd, Alanis Morrisette, Arcade Fire, Sloan, April Wine, Sarah McLachlan, Shawn Mendes, Metric, Loverbody, Streetheart, Barenaked Ladies, Anne Murray, Nelly furtado, Bruce Cockburn, Tate McRae, Michael Buble, Diana Krall, Blue Rodeo, Deadmau5, Avril Lavigne, Paul Shaffer and I don't know how many more that aren't coming off the top of my head right now. lol