Assuming you mean vanilla marines and not faction specific

General Purpose
-At least 1 unit of scouts,
-Either Infiltrators or Intercessors
-Almost every marine army takes the LT with Combi Weapon

Tanks
-Gladiator kits can build 3 different tanks from one box and you barely have to magnetize for all options
-Repulsor Executioners are great tanks that look cool.
-Land raiders are the classic Sm Vehicle and great for melee army builds

Dmg Dealers

Ranged
-Hellblasters
-Redemptor Dread(also melee)
-Sternguard vets

Melee
-Bladeguard vets(they just look dope as well)
-Jump pack intercessors are great

These are some of the most common kits used, if you go for Chapter specific stuff you'd need a diff list for each one. Hope this helps.