To clarify before we get started: These are positions Doomfist can leap from where he can deal a 125 damage Seismic Slam without needing to be knocked back or slide on a diagonal surface. There are extra positions on certain maps (such as Lijiang Garden) that would be on the list if they didn't require sliding, but these sliding leaps give significantly less freedom as to where you can land.
All Imgur albums are organised with two pictures. The first is looking at the position from somewhere the position can hit, and the second is Doomfist standing in the position.
Unless stated otherwise, all Slams were performed from a standing position. DO NOT use Rising Uppercut to get extra height! You will spend less time in the air and deal less damage as a result!
This high ground to the right of section B
The highest point of the castle in payload section A
This chandelier (you must jump for this one, you will deal 124 damage if you do not jump. DO NOT UPPERCUT) in payload section B
The three highest positions of high ground on Point B to the lowest ground in front of Point B:
1: The arch
2: The easiest-access one
3: The "Widow high ground"
The highest point of the choke on Point A
EDIT: The top of this building next to Point A
The catwalk above payload section A
The platform under the water tower Torbjorn players almost certainly use at the control point
Horizon Lunar Colony
This excruciatingly slow moving platform above point B
IIIIIIIIIN SPAAAAAAAAAAAAAACE (this is the only one where there is not a significant height difference between the starting and ending points)
This very high ground on the lighthouse
This very high ground on the building next to the well
On top of the broken pillars around the point (only provides enough distance if the destination is the control point itself)
This box on this high ground at the end of payload section B (only provides enough distance if the destination is the payload destination)
The head of the Mondatta statue
The highest entrance of the attacker spawn
Lijiang Night Market
The highest point of the buildings just outside the spawn rooms (almost completely useless)
Nothing! There is no ground on Lijiang Garden that is both high enough for a 125 damage slam and low enough for Doomfist to reach.
Lijiang Control Center
This high ground overlooking the point (you must jump for this one, you will deal 124 damage if you do not jump)
Nothing! There is no ground on Nepal Village that is both high enough for a 125 damage slam and low enough for Doomfist to reach.
Nothing! There is no ground on Nepal Shrine that is both high enough for a 125 damage slam and low enough for Doomfist to reach.
There is an elephant statue that is high enough to land on the low ground next to the control point with a healthpack on it, but you cannot control the Slam arc enough to actually hit the ground. You just leap off the cliff.
The top of these buildings outside the spawn rooms (almost completely useless)
Nothing! There is no ground on Numbani that is both high enough for a 125 damage slam and low enough for Doomfist to reach.
Ironic, isn't it? The map where Doomfist wrecked OR-15s in the airport is one of the few that he cannot perform a full-power Seismic Slam in unassisted.
Oasis City Centre
Uhh... inside one of the spawn rooms? (this one is so useless I didn't even put it in an imgur album)
This high ground overlooking the point (this is the most nitpicky one yet. You must both leave the blue railing intact AND jump before starting the Slam if you want to deal 125 damage. If only one of these is true, you will deal 124 damage)
This high ground overlooking the point (the hardest one to get into, as to get up there you have to Uppercut to the hole below it, then run backwards and jump, then Uppercut in mid-air to land on the high ground)
Nothing! There is no ground on Oasis University that is both high enough for a 125 damage slam and low enough for Doomfist to reach.
The railing on top of "The High Side" building in the middle of section B
This rock pillar at the start of section A
Temple of Anubis
This high ground in front of Point B (very popular Widowmaker spot)
EDIT: The similar high ground in front of point A (someone found a way up there)
The highest point of this building in front of the attackers' spawn (works in both directions)
This slightly less high point of the same building (only works Slamming towards the attackers' spawn)
The two highest points able to reach the lowest ground on section A
The highest point overlooking the end of section A (I actually forgot to get screenshots of this one, I'm afraid. It's the same high ground as "1" listed eariler, just closer to the end)
Pretty much the entire shuttle hanging above section B (the absolute front-most point is not high enough, but you don't need to jump from there the shuttle is huge)
This orange... thing next to the shuttle (yes i know it's an orange tarp but I don't know what's under it)
This incredibly high ground overlooking the start of section C
Don't do this on any KOTH maps or Numbani. Always try to do this on Watchpoint: Gibraltar.