I liked Marco's narrations best because he was a gift of a character and I felt like he got many of the wham books. Even when they weren't focused on the Visser One plot, they were still major (#10, #20). The only "filler" books that he got were #25 and #40, but I feel like those weren't as blatantly filler as #36 or #44 and I enjoyed reading them.
A lot of this comes down to personal preference, because if you really like Marco or think that Ax was hilarious, you're bound to enjoy their books the most.
I really liked Rachel's books early in the series and I've said why thousands of times. Cassie was my least favorite narrator because she's my least favorite character, but I enjoyed some of her better books like #29.
I also don't think that Tobias and Ax can be considered because they had half as many books. The only book I didn't like of Tobias' was #3, I can't decide which of his was his best because they all have his shining moments. Ax is also difficult to think about in this context because he goes back and forth between comic relief (#28) and being super-serious (#46). I think that's WHY people like Ax and while he's not as fun to read about for the younger crowd, once I read his books when I was older I LOVED them.
As for Jake, some of his books were EPIC (#26, #31) and some of them seemed kinda wasted. #36 is easily his worst and if it wasn't so early in the series, #11 would be up there too (it was too early in the series to really be considered filler). The scene in the Amazon where ants are devouring Rachel in grizzly morph is scary for that early in the series. I actually think that a lot of early Animorphs is pretty dark, but anyways...
It all comes down to who you like and who has the books with the best plots and characterizations. Rachel's characterization was inconsistent and Cassie got bad plots (#44 was obvious filler, #34 was badly-written, #39 could've been better, etc.), so that's why I think they don't get a lot of votes.