So, in summary, "the greatest space marine" doesn't work very well on the grounds that space marines belong to sketchy fictional organizations and it's hard to tell who belongs to what.
The Emperor -- Is he a space marine, or is he an emperor now?
X Shephard -- He's described as a "Soldier" or possibly one of the other classes, and a Spectre, but not a Space Marine.
Samus -- She's a freelance bounty hunter, really.
Master Chief -- We've already covered that. Really, nobody is specifically called a "Space Marine" in Halo, but they're probably all under a space marine umbrella (or do you think Space Marines wouldn't have ranks?)
Rico (?) -- There's a few Ricos in anime, and neither are specifically Space Marines.
According to Wikipedia, here's our choice of Space Marines:
- [li] Aliens Series (1986-1997) - United States Colonial Marines[/li]
[li] Babylon 5 (1994-1998) - EarthForce Marine Corps (also known as "Gropos" or "GROund POunderS")[/li]
[li] Battlestar Galactica (2004) - Colonial Marine Corps, Colonial Marine Corps Reserve[/li]
[li] Command & Conquer series(1995-present) Global Defense Initiative[/li]
[li] Doom series (1993 - present) - the United States Space Marine Corps[/li]
[li] Falkenberg's Legion series (1976-1993) - The CoDominium Marines[/li]
[li] Ground Control (2000) - Crayven Corporation's Marines unit.[/li]
[li] Halo Series (2001-Present) - UNSC Marine Corps, UNSC ODST (Orbital Drop Shock Troopers) Corps.[/li]
[li] Heritage Trilogy, Legacy Trilogy, Inheritance Trilogy (1998-present) - United States Marines Corps, United Star Marine Corps[/li]
[li] Killzone series (2003-present) Interplanetary Strategic Alliance Marines[/li]
[li] Lensman series (1937) - Galactic Marines[/li]
[li] Metroid (1986-present) - the Galactic Federation Marine Corps[/li]
[li] Outwars (1998) - Colonial Marines[/li]
[li] Quake Series (1996-present)[/li]
[li] Rolling Thunder - (2008) - Martian Naval Corps[/li]
[li] Space: Above and Beyond (1995) - United States Marine Corps Space Aviator Cavalry[/li]
[li] Space Marines (1996) - A United Planets Marine hostage rescue team battles space pirates[/li]
[li] Starblazers (1972) - Ground combact units found on the 11th planet.[/li]
[li] StarCraft (1998) - Confederate Marine Corps later succeeded by the Dominion Marine Corps. Other corps include the Alliance Marine Corps, the United Earth Directorate Powered Infantry and numerous more.[/li]
[li] Starfire series - Federation Navy Marine Corps[/li]
[li] StarFist series - Confederation Marine Corps[/li]
[li] Stargate Atlantis - US Marines are attached to the Atlantis Expedition as security forces, referred to as 'Space Marines' by Expedition members.[/li]
[li] Star Wars - Galactic Marines of the Grand Army of the Republic originally known as the 21st Nova Corps was an elite unit that was created as the Grand Army of the Republic diversified during the Clone Wars.The unit was commanded by Jedi General Ki-Adi-Mundi and Clone Commander Bacara.[/li]
[li] The Mote in God's Eye (1975) and related novels - Imperial Marines[/li]
[li] TimeSplitters 2 (2002) - Space Marines (Sergeant Cortez & Corporal Hart)[/li]
[li] TimeSplitters: Future Perfect (2005) - Space Marines (Sergeant Cortez)[/li]
[li] Traveller (1977) - Star Marines, Imperial Marine Force, Solomani Marine Corps, Zhodani Consular Guard[/li]
[li] Turok (video game) (2008)[/li]
[li] Warhammer 40,000 (1987-present) - Imperial Space Marine Chapters, and also to an extent Chaos Space Marines.[/li]
[li] Wing Commander Franchise (1990-1999) - Terran Confederation Marine Corps[/li]
Amongst those I'm familiar with from that list, I'm thinking the Warhammer 40k are probably the "greatest" at least in term of hardcore butch-ness. Genetically engineered, power-armored badasses. The only trouble is picking out a
specific one -- as I just covered, calling the Emperor a mere Space Marine is likely to get you lynched as a heretic. After that, you're pretty much looking at as many overzealous He-Men as grains of sand on a beach.