There isn't necessarily an easiest type to keep, and some of them can be extremely variable within themselves. It is better to ask yourself some more specific questions about what you are looking for in a pet snake. Do you want something small or something large? Even though temperaments can be generally good in some animals, there is no guarantee... can you deal with a large, powerful, aggressive animal? Do you want something with substance or something more slender? Do you want something that is nice for display or do you mind not seeing it that much or at all?
Just about all snakes are easy to care for, it's just more or less a degree of how hardy they are. Some certainly cannot withstand mistakes as well as others. It shoudln't be about what is the easiest, it should be about what appeals to you. Of course it is always good to get something that is relatively hardy to start with, but that is a broad array of animals. If you get something that doesn't really strike you, you may lose interest in it which does nothing for both parties.
As for "redtails"...
Once you do a bit of research on this you will learn that there are two subspecies of boas that are referred to as this. Colombian boas (Bci, Boa constrictor imperator) are commonly called "Colombian Redtails" or "Redtails". This is the incorrect term for them and was only given to them a few decades agoi as a way for dealers to squeeze some extra $$ outta them. There cousins the "true redtails" (Bcc, Boa constrictor constrictor), such as Surinames and Guyanese, were typically nicer coloured and patterned than they were. Bcc are said to be more sensitive to care for as well as in temperament. Now I have never put their hardiness to the test so I really cannot comment on that, however they do tend to be a bit more insecure temperament-wise I find. Both Colombian Bci and Bcc finish up in the 6-8 foot range on average and although they don't tend to look so, are incredibly powerful animals.
Either way, good luck with your decision