This is my first personal blog post, so i decided to give a summary of my personal views and observations about the Dragon Slayers abilities of the 1st Generation, and compare then to the other two Generations of Dragon Slayers. These are my personal opinions as to the abilities they have based off observation, so most of this will quite likely be conjecture; I gladly welcome any opinions or extra ideas or other ones entirely on my theories.
The First Generation of Dragon Slayers
Known First Generation Dragon Slayers:
These are the First Generation of Dragon Slayers; those who only obtained there magic from Dragons. All three were each found (presumably orphaned) by a Dragon, who took them in and taught them how to read, write, and use this Lost Magic. The Dragons who adopted these Mages are: the Fre Dragon Igneel (who found and raised Natsu); the Iron Dragon Metalicana (who found and raised Gajeel); and the Sky Dragon Grandeeny (who found and raised Wendy). They have the ability to "eat" so to speak the element that they are affiliated with (Natsu can eat fire; Gajeel can eat iron and Wendy can eat the air). They cannot enter Dragon Force on their own (apparently cannot at any length). However; they do appear to have have the ability (VERY limited ability) to eat the elements of other Dragon Slayers and use them in combination with their own. As of Chapter 318: Gajeel VS Rouge; only Natsu and Gajeel can do this. They use this apparent ability to create hybrid versions of their original magic.
The Second Generation of Dragon Slayers
There are only two known members of this generation.
Those of this Generation (Laxus and Cobra) were given access to these powers by having Dragon Lacrama implanted in their bodies. They were not raised by Dragons, nor did they learn their magic from Dragons. It is unknown whether or not they can enter Dragon Force.
The Third Generation of Dragon Slayers
They are called the hybrid generation, due to the fact they were BOTH raised and taught their magic by Dragons, and have Dragon Lacrima implanted in their bodies. They also claim to have killed their Dragons, in order to be "True Dragon Slayers". They appear to have the ability to enter Dragon Force at will. How they do this is unknown; though I am under the impression that they can because they have actually killed Dragons. The only known Dragon Slayers from this Generation are Sting and Rouge.
In Conclusion (Summary)
The Third Generation of Dragon Slayers does at first appear to be the strongest; seeing as how they have the abilities that they do. At first glance one could conclude that the First Generation would be second in strength and that the Second Generation would be the least powerful.
But, then take into consideration the ability that thus far appears to be unique to the First Generation (and has only been used by Natsu and Gajeel), the ability to eat the magic of another Dragon Slayer, and use it in combination with their own. The ability appears to be VERY limited, and can (evidently)only be used in extreme situations (usually that situation appears to be a life and death one). But, as evidenced by Natsu, if one can use this hybrid mode in such a situation, one could potentially use it at will. But, this almost always manages to instantly wear the user out (unless they have their Second Origin awakened). This in my opinion, makes the first Generation the strongest.
So in essence:
The Third Generation can enter Dragon Force at will.
The Second Generation is the one that the least is known about.
The First Generation can't enter Dragon Force at will, but in a corner can pretty much say: "Crap, this guy is strong, so I will just eat your magic and risk my life to power up; and beat the hell out of you." Then enter a mode of power that is quite comparable in strength to Dragon Force.