So, I get asked - a lot - both IRL and here, what the difference is between all the different kinds of Space Marines. What army should they take if they like X? Sure, if they like X, that's fine. But, there's a good chance that you can't base a whole army on X, and you probably will need some Y and Z to go with it.
It also occurs to me, that my previous guides all function like 3rd Ed. splatbooks and that you originally need Codex: Space Marines as well to get the most out of them. Which, isn't conducive to a lot of people who're asking me this question.
To that end, I've gone and built a list of units that 'work best' in any given particular Chapter. For this purpose, these are the units that you typically want to base your army around or have a lot (four or more, usually, or as many as you can get, slots willing), or, which units work best in conjunction with the rest of the units in that Codex.
The second point to the List, is to make What You Like == What Is Good. Which a lot of new players have a hard time doing.
Disclaimer: Space Marines is Space Marines. While you can build a Codex Marine army with 30 Terminators, a lot of people are going to wonder why you don't do it with Dark Angels instead. This is not a 'competitive' guide and will not tell you how to build an Army List. That is far more specific and you may be more suited to the linked guides above.
This is all subject to Cheesegear's opinion, and subject to change.
They are not written in order of preference.
Captains; Space Wolves, Codex Marines 
Librarians; Codex Marines (Utility), Space Wolves (Utility/Shooting), Blood Angels (Assault) 
Chaplains; Black Templars
Terminators; Black Templars, Dark Angels
Sternguard; Codex Marines
Dreadnoughts; Codex Marines, Blood Angels
Techmarines; Codex Marines
Tactical Squads; Space Wolves
Scouts; Codex Marines, Space Wolves
Assault Squads; Blood Angels, Black Templars 
Land Speeders; Codex Marines, Dark Angels, Black Templars
Bikes; Codex Marines (<1500), Dark Angels (1500<)
Devastators; Space Wolves, Codex Marines
Land Raiders; Blood Angels, Black Templars
Rhinos; Space Wolves
Razorbacks; Blood Angels
Drop Pods; Codex Marines, Space Wolves
This may also serve as an aggregate list of what you may want to take in your army, e.g;
Bikes & Scouts = Codex Marines
Bikes & Terminators = Dark Angels
Assault Marines and Dreadnoughts = Blood Angels
Librarians & Devastators = Space Wolves
1. Codex Marine Captains are the 'most' flexible of all Codecies, and, the special character Captains will literally change the way the rest of your army is played.
2. You will need a Librarian at some point. While many powers are the same across Codecies, they will function better in their typed roles due to the rest of the Codex.
3. In this case, 'Black Templars' refers to Crusader Squads, which are much the same as Assault Squads minus the Jump Packs with additional rules that make them better in Assault. Hence the inclusion.
I now submit myself to the
rage criticisms of the forum.