I’ve been messing around with this style for a while on ladder and found it fairly effective. My opening of choice is a standard reaper opening as it is safe and can give you decent scouting intel while allowing for a very fast stim. This style requires good multitasking and marine split micro to be successful. I'm currently sitting at 850 points in masters league and have played TvZ against other high ranked master and GM zergs.
General Build order
12 rax 13 gas reaper opening
CC while reaper is building
Stim when you get the gas
Add 4 rax before CC completes (don't get a 3rd supply depot as your cc finishes as soon as you get supply blocked.
Make marines and scvs nonstop and add barracks if your mineral count starts to go up. (Add marauders if you see roaches or infestors at any point)
I won't go too far into the build as it's fairly flexible. The main goal is to make a lot of marines
Fast 3-3 upgrades – It is unlikely that you will finish the game with the first attack. If zerg skips out on upgrades, marines become much stronger against lings and you will win battles that you would normally lose.
Optimized mineral vs gas income – By staying on a low number of refineries for a long time, you get ahead of the zerg in mineral income.
Nonstop scv production – in order to maximize your mineral advantage, you need to stay even with the zerg in worker count.
A lot of barracks – Add rax when you start to float money. You should have at least 10 rax (or equivalent with reactor or tech lab) with a reactor port and double ebay if you're on 2 base 2 base.
Denying creep spread – Since your aggression starts quite early, zerg often does not have much creep spread going when your first attack hits. Try to save up a scan so that you will be able to clear any tumors. It might not be helpful immediately but your next wave will enjoy the reduced speed of the zerg army. Wherever possible, engage off creep.
Flexible transitions – Because you power on economy especially if you grab a fast 3rd, you have the resources to transition into any gas-heavy builds such as tanks, ghosts, bcs, etc.
Relentless aggression - The idea is to constantly put on pressure and force zerg to make zerglings and banelings nonstop which means no more drones while you are building scvs one by one. If zerg overdrones, he can end up losing a lot of drones and is put into an unwinnable position. Thus, you are forcing zerg to either walk a tight rope or play safe and inevitably fall behind in economy.
Macro and micro - You need to push your multitasking to the limit so that you have the most number of marines to attack while getting the most out of each and every marine.
Compared to a more traditional marine/tank/medivac army, mass marine’s aggression scale is off the charts. It gives a huge tempo advantage and forces z to be on his toes the entire game. With the huge mobility of your marine/medivac army, you can do multi-pronged attacks, hitting several locations at once. With good micro and multitasking, you can pull yourself way ahead of the zerg.
It’s not just zerg that is under high pressure. One misclick can cost you the game, especially in your earlier attacks as it gives zerg ample opportunity to drone up before the next attack if your entire army dies to a few banelings and zerglings. Later on, infestors can pose a huge threat to your army if you’re not careful as 2 FGs can kill a good chunk of your army in about 5 seconds with no way for you to micro out of it.
Responses to specific openers
Roach rush: You should be able to see this with your reaper. Bunker at your ramp and start marauder production asap. Start concussive shell so you can put on a huge amount of pressure when zerg starts to retreat.
2 base bling bust: This is one of the biggest threats to the mass marine build as it hits before Terran has a big production going. If you got stim asap, you should be able to micro and take out all the banelings. If you catch wind of this rush coming, double bunker at your nat and push out. You want to trade as cost efficiently as possible with the banelings and use your reinforcing marines to defend with bunkers. If you don’t have stim, you need to split all your marines into groups of 3 at most and stand your ground by the bunkers.
Fast infestor: There are several responses to this. The simplest is to start adding marauders. You can go for banshee into BC to make zerg spend all of the infestor energy on killing those instead of the mmm ball. Preemptively split your army before moving anywhere so you don’t get caught by surprise with one nasty fungal (into infinite FG -__-).
Ghost: Ghosts get the infantry upgrade bonus which means you don't need to get other upgrades and you should already have some tech lab barracks to make ghosts and once you grab all your gas geysers, you will have plenty of gas to produce ghosts even with double ebay and reactor port.
Raven: This style can be good if you have enough advantage to buy you some time until you build a sufficient fleet of ravens. However, the huge amount of resources that you end up spending means your army will take a pretty big hit and you are left rather vulnerable for some period. Assuming that minerals are worth the same as gas, ravens are essentially 6 marines that can’t attack for a long time and requires a ton of upgrades to be useful.
Tanks: Tanks are the counter to the two biggest threats to marines – banelings and infestors. Although your mobility will take a hit, 3 fac tanks can work wonders if zerg is still on pure ling/bling.
Banshee into BC: As mentioned in the above section, this unit transition is great for zergs that skipped mutas for infestors. Two FGs on your banshee means 10 less dead marines on your side. BCs are a huge pain to deal with for a spireless zerg as their only real anti-air would be queens and infested terrans (which costs even more precious energy).
Most of these replays should be the opening described in this post but don't hate me if there's some odd replays here and there since I tried to pick them from memory!
I know this build still needs a lot of refinement so please let me know if you have any suggestions to changing up this build and I'd be happy to try and incorporate it into my play
Marine splitting trick by Mjoa
Marine split challenge map by Griffith