I'd say widow is your best option: 3 shot with widow, but after first shot bastion can be reactive and try to run out because he know that if he wait 2sd shot to go back to recon form (which take 1 second (0.5 to go sentry)) you'll have charge the 3rd hit.
To rephrase, bastion HAS to recon once a widow shot him once, or his dead (same go with phara and hanzo) but if he do so he can survive, (especially if he placed himself next to a wall). So if he recon after first shot you need the headshot. It's really easy to headshot Bastion from long distance with widow because of his big head and slow movements. With hanzo it would be a little harder if distance is really big. And phara... well headshot, you know..
So definitely, for me widow is the best option to kill bastion -> 3 shot if he don't recon, 2 shot if he recon and you get the head. You don't even need a reinhardt if you have a wall you can hide behind after each shot, you kill him first, but even behind reinhardt I don't see a better option.
Remember that bastion shot 30 bullet doing 15 damage each second so 450 damage per second, reinhardt shield has 2000 hp, it will take about 4.4 sec to destroy reinhardt shield, if they have a second bastion or phara or something doing huge damage to reinhardt shield, it won't last enought time for you do destroy it, or even if you kill him, reinhardt won't have more that 650 hp (assuming only one person try to kill bastion, it will take 3 second, for widow phara and hanzo) So you'll be in a bad position even after killing one of them. So you'd better kill it with widow or make him go back with phara or hanzo without reinhardt but hidding behind a wall after each shot, so you save reinhardt shield.
Assuming you really want to be behind a reinhardt, I think (sorry for mistaking earlier) you'd better go for a bastion which will destroy him in 1.2 sec ( 0.7 sec of damage + 0.5 sec to get in sentry) so here you save your reinhardt shield at at least 1400 hp. You can even forget about the 0.5 sec to get in sentry because you can do it while jumping and reinhardt can shield you once you are in sentry mode (assuming you won't die in a jump of 0.5 sec getting sentry, since bastion need about 0.7 sec to damage you enought + you have some movement). So behind a reinhardt, Bastion is the best because you NEED TO think about your reinhardt shields hp. Still you'll have to attack with a bastion after that, I still think widow without reinhardt shield's is the best option.