Terran Reaper Build Order

Solution 1:

Generally, a fast Reaper rush is most effective against Protoss. Against Terran, they may have 2-3 Marines before your reaper gets there, which will make it ineffective. Against Zerg, you will often have to deal with a Queen or multiple Zerglings on creep. Protoss, though, requires a Cybernetics Core along with a Stalker to be able to combat it.

Here is the Turbo Reaper Build most commonly use against a Protoss opponent.

Another option against Zerg is to use a build that LzGamer has been using lately, which is to build 3 Rax quickly and pump reapers from all of them. This applies huge pressure because the Zerg opponent can't take on large groups of Reapers without speed upgraded Roaches. I don't have an exact build for this one, but there is some discussion by Zerg players about how to combat it here.

Hope this helps some.

Solution 2:

The thing about reapers, is that they're fairly predictable (in terms of a good opponent anticipating it if they know you're terran).

Against any decent player, they'll ready a Stalker, Marauder, or Queen to handle any reaper harass. That means you have a relatively small window for the rush to be effective.

If your enemy has stalkers / marauders by the time your reapers reach their base, you've taken too long. What's your general build order? Check your replays -- what's the game time when your first reaper comes out? What maps are you trying to do this on?