If affordability factors into what would be the “best” then you can’t beat the 24-volume Early Church Fathers, edited by Philip Schaff.
The set is now in the public domain (so it’s free) and can be accessed (and electronically searched) here:
Schaff compiled the volumes from various translations (he is not a translator himself). He is a protestant. I do not find that he has tampered with any of the texts, but his commentary shows an obvious protestant bias. I stick to the texts, and rarely consult the commentary.
An additional collection, edited by Robert Pearse, can be found here:
There is a lot of overlap between these collections.
I highly the three volume Faith of the Early Fathers by William Jurgens. These are books whose value is not diminished by Google or Wikipedia, because they have something that no search engine can offer: a Doctrinal Index. This is what gives this collection it’s real value.
Suppose you have a protestant who says that Baptism is merely a public ceremony giving witness to a salvation event that has already occurred (when he “accepted Jesus as his personal Lord and Savior”). You, on the other hand, maintain that Baptism is the ordinary means by which we are saved (our conversion experience might lead us to Baptism, but salvation comes through Baptism, not through the prior experience).
What does the Early Church have to say about it? Flip to the Doctrinal Index. Under the section for Sacraments, you will find Baptism. You will see that there are seventeen different Baptismal doctrines covered (including, for example, that the Baptism of John was not a sacrament). Scroll down the list, and you will see:
The effect of Baptism is spiritual regeneration, which consists in the remission of every sin with the punishment due it, and the infusion of first grace.
You will there find forty-nine numbered passages from the Early Fathers which support that doctrine. In the main text you will find those numbered passages, all of which relate directly to that doctrine.
You will occasionally find passages which contradict that doctrine (those would be in parenthesis). The Early Fathers were not unanimous. They happened to be unanimous on the topic I mentioned, but on the question of "Even heretics can Baptize validly (#830) there are eight passages contrary to the doctrine, and sixteen supporting it.
I have no idea how you might go about finding these passages without such an index. You might find a few with Google (along with hundreds of irrelevant links), but not 49 - not even close.
Jurgens also gives helpful biographical information on each author and information about each individual text.
Jurgens rarely includes the complete text of any documents - these are only doctrinally relevant extracts. If you want the full text, you can refer to one of the collections I indicated earlier.
My superhero identity is Kaptain Katholic, and this is the source of my powers.