Titanfall

You can earn up to 3 stars per gametype per map, which gives a total of 504 stars (though I'm not sure you can play marked for death on the base maps right now, so that lowers it to 459.)
I'm sure 3 stars in every single gametype isn't going to be feasible for most people, so I'd say honestly just shoot for at least one star in each gametype/map and anyone could easily earn this.

Star requirements (1/2/3)
Pilot Hunter - 5/10/20 Pilot Kills
Hardpoint - 500/1500/2500 Assault Score
Attrition - 25/75/150 Attrition Points
Capture the Flag - 1/2/3 Flag captures
Last Titan Standing - 2/5/8 Titan Kills
Marked for Death - 1/4/7 Marked Kills (To me the hardest to get 3 or even 2 stars for.)
Frontier Defense - Map dependent - Generally fairly easy to get 2 stars but requires good teamwork to get 3.

If you wanted to focus on one gametype to get most of your stars, I'd recommend Frontier Defense as getting 2 stars pretty much only requires not letting the tower die. (And you may honestly be able to get 2 even if it does.)
Also as xPut Name Herex reminded me, you need 2 stars in 15 frontier defenses for another achievement, so it might be best to start with those.
One star on any given gametype should easily be feasible for most Titanfall players.

an easy way i found to do this and getting the 3stars for both players is playing CTF on an empty server from the main with X then look for a match and you'll end up in an empty lobby with your friend. both of you get three flags one get the fourth and the other person backs out you get a win and the three stars and they get the three stars too