What pack to start with?
Understanding what's most popular is just one way to start your search. What matters ultimately is that you know what to consider to find what best suits your preferences and needs.
The most popular starter kit
With a large enough range of shapes and a decent number of pieces, the 100 something packs are generally the most popular sets to start on for one kid. Beware though, 99% of people will tell you that you are likely to need to add to it quickly and you can never have too many magnetic tiles.
Builds? Wheels? Ball runs or mazes?
For some people, magnetic tiles are mostly about that one thing the child is really into. Obsessed with wheels, cars and trains? Look for car packs and car bases! Love tracking the ball in a crazy run or maze? Ball runs are for you. Already a fan of Lego and Duplo? MagBrix compatible tiles are your answer!
Shapes and colours
Is this primarily for making rockets and skyscrapers? You may need a square Expansion Pack. More into detailed, sophisticated cottages? A Mega Pack will have all the shapes in all the colours of the rainbow. Are you more into mazes or ball run? Base plates are a must, especially on carpet and soft flooring. Is making full circles (dodecagons) with your isosceles triangle pieces essential? Then go MagBlox 101 piece set.