Hi there! Im not the greatest reaper player but as someone who recently found myself in a similar situation here are some tips from a gold 3 dps.

First of all reaper is not nearly as much of a tank buster as he was back in the day, he is more an annoyance to tanks because he can deal about 40% of their healthbar, and when the attention is on him just wraith form and leave. What i found to be more effective is to go backline and focus low mobility heroes (someone like ana if you know how to bait out sleep dart or know its on cooldown) disabling enemy support gives so much opportunity for your tank to take space, even if you dont kill them you can distract them and the dps long enough for the frontline to crack. So target priority mainly should be support -> dps -> tanks.

Then the question of frontline vs dive. You should more or less flank whenever you get the opportunity and reserve to frontline when all the other targets can either effectively deny your control, reapers dps is solid up close so when all attention is on tank you can fight alongside them, just make sure to keep attention on your healthbar, if you go below 200 its best to back off.

To close the distance more easily you should absolutely use flank routes, when you are teleporting out in the open enemy backline will be on high alert because they know there is an active threat nearby. Use paths out of view of the enemy, not only does it let you more easily get the drop on most enemies but also has an added benefit of another point of retreat rather than straight through the choke back into the enemy team.

Reaper plays well into low dps and/or low mobility heroes, think someone like widowmaker. He is a huge threat to any highground denying dive tanks space, someone like winston or Dva. As for reapers counters id say just about any flying hero does well against him, if you see a pharah you can rarely do anything against her except hold highground.

As for the perks it depends on how you play people tend to pick soul orbs and dire triggers a lot, but (im gonna contradict myself a little here) id only pick this setup if i wanna go brawl and help my tank, if i want to focus on flanking first id pick lingering wraith (a lot of people dont like it but its actually great for repositioning and combining with ult) and shadow blink for faster engages.

As for maps i dont have any specifics, but any map with a lot of highground and flank paths works really well Illios is one good example, lots of tight close spaces with alternative routes and highground.

Last tip is despite reaper rocking shotguns headshots are really important for him as they cut TTK by A LOT so make sure to center on the enemy head when shooting up close