I'm torn between Gavan and Sharivan as my favorite of the trio. I actually dislike Gavan's costume the most, shockingly, despite my views on Shaider's design.
But that's almost purely because his suit looks so...soft and...not metal. It looks almost plush in places, which is a stark contrast to, say, the stock footage of him using the Gavan Dynamic, which shows him in glorious gleaming metal armor.
However I FREAKING LOVE his battle ship. SPACE TRANSFORMING LASER DRAGON.
Meanwhile, while I'm not crazy about Sharivan's chest (Little too boxy) I adore his helmet, and he's a pretty interesting character. ...a horrible police man, but an interesting character. He's got the strongest costume, I think.
I think ultimately I have to point towards Gavan, if only for the running subplot of Hunter Killer, his father, and all that connects there.
Sharivan didn't really HAVE an over arching plot like Gavan.
Shaider I sadly haven't gotten to see past the first episode, so my views come strictly from that. I can't speak for its storyline, and I can't speak for later episodes. But what I saw was both trying to be serious and very very obviously goofy at the same time, clashingly so, and his design has never worked for me.
Partly the helmet, namely the visor, and partly the color choices. His legs just...look off to me.