G0-Onger IMO is easily the most childish Sentai series with the talking mechas complete with anime eyes (the engines), cartoon strips, and its comedic style. Commercially Go-Onger was a success, selling 12 Billion Yen in toys, the highest since Abaranger. However, this strategy alienated older adults. Among English-speaking fans Go-Onger has come off as a love it or hate it series with many calling Go-Onger the worst Sentai series, mostly due to its childish nature. Personally, I don't think Go-Onger is a bad series at all. Actually, I feel that its childish nature works well in its favor and makes the show entertaining. The engines have really cool and vibrant personalities with my favorites being Speedor, Birca, and Bear RV. As for the show's comedy, I thought it was ok. Not as good as the comedy from Dekaranger, Boukenger, or Shinkenger but much better than say Goseigers, Power Rangers Ninja Storm, and Power Rangers Samurai.
Magiranger I think is the second most childish Sentai behind Go-Onger because of the magic motif, characters like Mandora Boy and Smokey, and its goofy comedy. However, this series mixes its childishness with some pretty dark and serious moments. Unlike Go-Onger, Magiranger doesn't get as much criticism for its childish nature due to its unique story. Personally I think the childish aspects of Magiranger were ok. I'm not a fan of magic shows but I felt the magic motif overall was handled ok. Mandora Boy was ok and I actually liked Smokey quite a bit. The comedy, like Go-Onger's, I thought was just ok. However, for me Magiranger is at its best when its more serious, like when the story focuses on the villains or Magiranger's family aspects.
Zyuranger is another childish Sentai. The dinosaur motif was handled in a fairytale fantasy setting, the child of the week gimmick, and its kiddie outro. This seemed to work well with kids but like Go-Onger it alienated older audiences. Among Western fans, Zyuranger has fans split between fans who think it's not that good and fans who tend to overrate the series due to MMPR nostalgic factors. Personally, I disliked Zyuranger's childish aspects. I don't really care for the fairytale setting in Zyuranger, but unlike Magiranger I didn't think it was handled well at all. Also, the child of the week gimmick I feel is pretty lame and distracts the show from character development. However, like Magiranger, I think Zyuranger is at its best when its story circles around its more serious aspects such as Burai's touching story, his relationship with his brother Geki, and to an extent Bandora.
Goseiger is fourth on this list for a few reasons. The show's motif IMO is pretty childish with the angel motif and the second outro was kinda kiddie. However, it's last on this list because when you actually watch the show it seems unsure as to rather it wants to take the turn of childish comedy like Go-Onger or go for the more serious tone. Goseiger hasn't been well received among Western fans. I have to admit like Magiranger's motif, I'm not much of a fan of Goseiger's angel motif. But at least Magiranger did handle the magic motif reasonably well, here Goseiger does virtually nothing with the angel motif. Initially it was set up well with the 5 Gosei Angels from three different tribes but other than having different powers and abilities, nothing is really done to distinguish how these tribes are different. In Zyuranger for example, when all the characters are from different tribes, we do at least get some background story, particularly surrounding Geki and Burai. However, unlike Magiranger or Zyuranger, the serious moments in Goseiger aren't that well handled IMO, at least not until towards the end a little bit. So for me, this series fails both with its childish and comedic tone, and its more serious moments. Goseiger so far is the only Sentai series I truly dislike.