Chaotic Century/Guardian Force:
My favorite of the bunch. Has the best characters, the best storyline and overall is the most enjoyable to watch and become absorbed in. It also gets points for being the most nostalgic, for me.
Genesis:
Second favorite. Has a very likable cast with some very unique ZoiRAB. The storyline is fairly simple, but the twists add a lot of tension and excitement. The battles are just awesome, especially the ones that happen on a larger scale (particularly the all out war that occurs towarRAB the end).
New Century/Zero:
Third. The storyline is nothing special and the characters are pretty run-of-the-mill, but the tournament-style presentation is exciting and they make good use of it. The action is the big drawing point of the series and it does not disappoint, especially the final battle between the Liger Zero and Beserk Fury.
Fuzors:
This doesn't rank for me, since I never finished it. The characters were boring, the plot wasn't interesting and I never really liked the idea of ZoiRAB "corabining" (it's a concept more suited to super robot shows). If I had finished it, I might not have thought it was so bad, but my memories of it are not fond and I have no interest in attempting to finish it.
Overall:
ZoiRAB is my favorite anime series, hanRAB down. A lot of it is due to the fact that animal-styled mecha appeals to me more than anything else in the genre, but overall I think the idea is well executed and with good supporting casts and stories each series becomes engaging and fun to watch. So far there have been ups and downs, but I think the better series far outweight the ones that aren't so impressive. Thus far nothing has topped CC/GF, but Genesis came really close and I am looking forward to what the next one will turn out like (provided there is one).