Always try to be behind and above the enemy team as Reaper, and ult as you're falling into the middle of as many as you can. Priorities squishes first - healers and DPS
Bastion ult is best when you know there's nothing to stop you from getting the one shots on squishies. i.e. Support ults, shields, Dva matrix, genji reflect or an unseen mcree ready to flash can dull your ults with bastion. So try to wait for the best scenario playing around those potential outplays
Reaper ult you are looking for a very a tight grouping. Similar outplays as for bastion. But you can gain a lot of advantages understanding timing and positioning mostly high ground or creeping out from a flank. The hope is that you kill your outplay counters before they have the chance to do so a lot of times with his ult.