The Marauder is an easy character to learn and master in Path of Exile, especially with the kind of Path of Exile items it requires being rather straightforward. The only thing that gives variance though is the build that a player uses for his Marauder. Despite the perceived simplicity of the character class, there is actually a good degree of variety in builds for the Marauder, so here are some of them to help you decide on which is best for you.
First off, let's start with a bit of a curve ball. There's actually room for a Bowrauder, which seems unusual for a character that's supposed to excel in melee combat. However, this build is actually quite versatile and is good for solo play due to high damage output and survivability.
Your main skill gem for this build is Rain of Arrows, which is like a shotgun blast of awesome. Your passive skill points are to be mostly spent on life and armor and none on accuracy, so Rain of Arrows is a perfect choice here. You then support it with Auras like Wrath, Anger, Hatred, Purity, Determination, Grace, and Vitality to choose from.
Basic Melee Marauder
Of course, there's the fundamental Marauder that likes to get up close and smash enemy skulls in. This build is best for new players who are just starting to get into Path of Exile and may not have a lot in terms of PoE items to go around. Being balanced on both offense and defense, this is a good choice build for most players who may not know their way around yet.
Your main weapon should be a 2-handed mace, which seems like it's meant for the Marauder to begin with. The main focus in stats is strength, which makes up most of your melee damage. With active skills, you can choose from Ground Slam, Leal Slam, Heavy Strike, Endurance Cry, and Warlord's Marks to supplement your character in a variety of ways.
This is quite unique build that has great potential in being very strong, as long as you have the right gear. The playing style entailed by this is rather boring for most though, but that's just because you'll be so hard to kill if you make use of this.
Your main skill gem is Ethereal Knives, which becomes better if you get either Life Leech, Faster Projectiles, Added Fire, Iron Will, and Faster Casting. You can also use Bear Traps with Added Fire and Culling Strike to make it better as well.
Hardcore Two-handed Marauder
This one is specifically designed for full-on hardcore play, so the main point here is survivability above all else. You'll be using a two-handed mace, and the main skill gem is the Glacial Hammer, complimented by Weapon Elemental Damage, Melee Physical Damage, and Faster Attacks.
You can also have others like Ground Slam, Molten Shell, and Enduring Cry, as well as either a Rejuvenation Totem or Decoy Totem for better defense. Playing this build requires patience as going crazy on offense will only get your killed. Slow and steady is the name of the game with this build, so you'll have to pace yourself.