Methyl halides such as CH3Br/CH3Cl/CH3I, etc. are most suitable for Sn2 reactions because they are less sterically hindered by R-groups (they are not "bulky"). This allows for easy attack by the nucleophile.
Primary alkyl halides (RCH2X) are also most suitable for Sn2 because of the same reason above
Secondary alkyl halides can undergo both Sn1 and Sn2 reactions, this depends on other factors such as solvent and leaving group and nucleophile. If the solvent is polar aprotic, the reaction will go Sn2, if polar protic - Sn1.
Tertiary alkyl halides (alkyl halides with 4 r-groups) do not go Sn2 because they are bulky and the R-groups stabilize the carbocation by hyperconjugation and inductive effect.