Translink has a list of performance metrics per bus route here. Those include ridership, boardings per revenue-hour, crowding measured as a percentage of available seats, and operating cost per unlinked trip. Since the numbers are only given per route, without a single table or chart as one could find for Providence or New Haven, here are the busiest routes, per weekday:
1. 99 – 54,350
2. 20 – 27,900
3. 9 – 25,300
4. 41 – 24,800
5. 16 – 21,250
6. 8 – 20,150
7. 3 – 19,950
8. 49 – 19,700
9. 135 – 19,600
10. 25 – 19,300
The full sanitized data for daily and annual ridership, excluding minibuses and night buses, can be found here. I’ve verified that excluding minibuses and night buses doesn’t change the rankings in the top 50 routes.
Although Vancouver’s buses more or less run on a grid, the grid isn’t very clean. Some lines, like the 9 (Broadway), 99 (Broadway), 3 (Main), and 41 (41st), run more or less straight north-south or east-west, bending only at the ends, but many others do not. The 16 follows a broad U-shaped route, serving Arbutus on the West Side, feeding into downtown, and then going east on Hastings and then south on Renfrew. Multiple routes use Broadway for just a few blocks, to orient themselves to the correct north-south street. Others are L-shaped.
This makes it hard to figure out what the busiest corridors are (Vancouver has enough ridership that the 15-minute frequent network extends too far down to give us the busiest routes). Broadway is clearly the single busiest – if 99 and 9 are considered express and local versions of the same route, then Broadway has nearly 80,000 weekday bus riders, compared with 55,000 on 1st and 2nd Avenues in Manhattan, without counting buses that serve small segments of Broadway along their trip. Not counting buses that zigzag, the next busiest are 41st Avenue (41), Hastings (135, 160), Main, and 49th (49).
But this partial interlining does exist. So how busy is Hastings, anyway? If we add the buses that go on inner Hastings – 14, 16, 20, 135, and 160 – we get 90,000 weekday riders. But the 14 and 16 have half their route on the West Side, and the 20 turns south on Commercial; those are not just Hastings buses. The same problem happens on Main (the 8 partially runs on it), and 4th (west of Granville it interlines the 4, 7, and 84, and west of Macdonald also the 44, totaling 40,000 riders).
This doesn’t mean Hastings has more people riding the bus on it than there are taking the Millennium Line. I doubt it’s even close – the 16 and 20 have long north-south legs with connections to the Expo and Millennium Lines, so people from Fraserview and most of the Renfrew corridor are probably not traveling anywhere on Hastings. But most likely, whatever fraction of 90,000 Hastings has, it is probably the second busiest corridor, or maybe the third after 4th.
The obvious problem here is for SkyTrain development. Broadway is almost certainly getting rail, and judging by how far lesser-used corridors are getting SkyTrain extensions, Hastings should get one too. 4th is half a kilometer north of Broadway, but Hastings is 2 km north of the Millennium Line. Hastings’ distance to the West Coast Express is shorter, but it is an active freight line, with active port industry to its north, and often parks separating it from the street grid to the south. Frequent, frequent-stop commuter rail is still possible, but half the station radius is wasted on water, and the freight traffic is such that it might require too much multi-tracking to be cost-effective for the potential ridership.