From Halopedia, the Halo wiki
Steaktacular is arguably the most easily earned Legendary medal, and is most easily earned in Big Team Battle matches, as winning by 20 or more kills is much easier in 100-kill matches than in 50-kill matches. Triple double is likewise more easily earned in in BTB, and a similar argument could be made for Unfriggenbelievable and Perfection. The only medal which is irrefutably harder to earn is Extermination.
If the player is sufficiently skilled, they can earn many Killionaire and Unfriggenbelievable medals in Infection.
With the addition of Halo: Reach Firefight, players earned another arguably even easier option for earning the achievement, as Killionaire medals earned Firefight matchmaking count toward the achievement. Unfriggenbelievable medals earned in Firefight for killing 1000 enemies without dying are not considered Legendary medals, unlike the medals earned in multiplayer by killing 40 opponents without dying (though the distinction is effectively moot as no Firefight matchmaking modes contain 1000 enemies).