While there are some Sentai mecha designs I have not much of a comment on, here are those I think are really cool:
Turbo Builder- I am amazed at this base robot!
Max Magma- After the Magmabase goes robot mode, I really find this cool!
Great Icarus- It has this really cool design I just can't explain why.
Hyper Haken- Just the coolest jetplane in Super Sentai ever!
Dragon Caesar- I like it for its tribute to Godzilla design.
Kyokyoku Daizyujin- I like it for its huge cannons and rider formation.
Dairenou- I'm just SO in love with the armor design.
The Heavy Armor Chi Palace- I like its unique design for some reason.
Dekabase Robo- It was the first base robot to walk and also it has plenty of awesome weapons.
Dekawing Robo- I like its design especially to how it can become a giant bazooka.
Ultimate Daibouken- So far from all the multiple mecha joining together, this is the coolest.
Mougyudaioh- I think this is Shinkenger's coolest mecha. The rest IMO is not really that cool.
Samuraihaoh- I like how the eleven mecha of Shinkenger combine into this one awesome powerhouse.