Of the frogs you listed.... I'd stay away from the lamasi and imitators as first frogs (they are members of the thumbnail group and I don't recomend any for first frogs). Imitators are great for first thumbnails, but they are so tiny, even as adults, it might be better to go through your trials and tribulations with a larger (and cheaper) species. I wouldn't even consider pumilio right now, not until you got a number of years with tinc group and thumbnail groups so you know what you are doing frog wise and can make sure you can raise the small froglets if you are lucky enough to breed them.
Tricolor are probibly the hardiest darts I've come across... easy to breed, feed, raise, etc. Also very bold (and loud compared to most of the tinc frogs). I've always recomended these as "the" beginner frog, but after falling out of the hobby a couple years ago, most people started out with tincs, auratus, and leucs (thus their high recomendation rates). They are easy (will even eat hydei, in fact seemed to prefer them even with their small size) and will get you used to smaller frogs, especially if you are thinking about thumbnails someday. If these guys can live and reproduce in grassland areas using puddles, they should have no problems dealing with a new frog tank as you get used to keeping temps and himidity up, etc. I can't recomend these frogs enough to beginners, but I guess I'm one of the few thats been doing darts long enough to remember how common they were in the past. Dart frog availibility and price effect their beginner recomendedness a lot of times, and with tricolors not nearly as common as tincs, etc, they tend not to be recomended enough, but they are around and for about the same prices as tincs and leucs.
Tincs would be my second choice due to their larger size and boldness. Leucs would also be good, but I have to say I always did enjoy the tinc personality more... like when they begged for food, lol. Both species are readily available, and with tincs I'd recomend a cobalt (any of the various cobalts would do).