Any alternatives to mega blocks and stickle bricks?

Duplo beats all the others and lasts generations. Fab from 18 months to 5 years.

Duplo. My son is playing with my childhood set. It lasts forever

I have found Duplo(lego) is the best construction toy for preschool children. Much better than the 2 you mention.
For 3+ mine also love magnatiles and mobilo.
All three are expensive but are worth buying second hand or collecting small amounts of.

There’s Tegu but it’s really expensive

Duplo is brilliant.

Definitely get the sets with bricks rather than loads of pre moulded other parts to start with.

Lego and Duplo without a doubt.

Magna tiles?

Magna tiles are expensive but brilliant.