I have to agree with Jakob, anything ( I always say this to everyone asking the same question) if you actually bother to research them is "Easy" to keep, however there is no easy in my opinion, you make it easy for yourself if you research them. But by saying Dendrobatid, you are talking about approximately 178 species in total some ranging from "Expert" to "Beginner" so you need to be slightly clearer as to what genus your thinking about keeping/researching lol
Most species from the Dendrobatidae family are quite simple to keep but like CG and Jakob said E.tricolor (Phantasmal Poison Frogs) are very simple to keep, I would also recommend as a first species Hyolaxus azureiventris (Sky-blue Poison Dart Frog), Dendrobates leucomelas (Bumblebee Poison Dart Frog) or even a Dendrobates Tinctorius (Dyeing Dart Frog).
I really dislike calling anything a "Beginner" species because even the most hardy species can die if the person doesnt bother to actually research into the care of the animal lol.
You can find Tricolor's at very very cheap prices, I have seen £10 each before now, even cheaper than that sometimes! But having kept them for a long time they are a great species regardless of being very simple to keep, highly recommend them to someone wanting to start out with PDF's. I would say keep them in a group as I think they prefer it by far, I am getting another 3 when I can for that reason lol but a group of say 6 could easily live happily in a 2.5ft x 1.5ft x 2ft Tank (Length x Deep x High).
Of course it's all your choice of what you want to keep, you shouldn't get something because that's what some one recommended.
I think some Mantella species are simple to keep aswell, however I would say start off with PDF's and get experience with them before you start with Mantella. I think my M.aurantiaca are fantastic, my other frog species I have kept or keep are not nearly as bold (Even my Tricolors have never been as bold)