I have worked with all three:
Zane: They were the last big hold out that reimbursing individual premiums was legal when it clearly was not. Obviously there have been some changes now. They took a bold stance but did not back it like the other TPAs financially if a client got left holding the bag.
Base: nice people, responsive, and reasonably priced. The only red flag was that I was teaching the rep about the recent HRA developments a few years back and they didn't appear to be keeping up with it closely. Maybe I am wrong on that assumption.
TASC: big player in the market, I have confidence they keep up with all the changes and updates in their documents. For small plans, they can be expensive. Customer service seems to be improving, that was and is still my biggest knock against them.