Hasbro Playskool Busy Gears
This probably isn't in his top five anymore, but it's notable because I can't get over how evergreen it has been. His Grandma Kath sent it to him about a year ago if I'm not mistaken, and it has been among his favorite toys since the day it arrived. I'd say his interest in it peaked around 15 months. I would never have pegged it as an engine of inventive play, or a toy that would have unfolded over the months with different ways of interacting with it. It looks so one-dimensional!
You push the button, and music plays while the center cog turns, which turns the other cogs. The center one is attached to the toy; the others are removable (and stackable, though that doesn't seem to add anything to his enjoyment).
When he was tiny, he just liked taking all the cogs off and chewing on them. I would push the button for him occasionally, and he liked that well enough that he learned to do it himself. (And the music isn't bad, for a mass-market kid toy -- it might be the least annoying music-making toy in the house.) He then went through stages of mastery of manipulating the cogs in various ways: putting them back onto their nubs, sticking his fingers in the way so they can't move (it makes a different noise when the cogs are blocked), etc. Lately his interest is in putting things on the main cog so that they spin around or fall off. He's been experimenting with that for a while now.
It's like he woke up one morning about a month ago and suddenly had peg puzzle mastery. He'd had a couple of the really big chunky ones for a few months, which mostly got handled and chewed and their pieces misplaced, but he never showed much interest in "solving" them. Then one day I got out of the shower and found him poring over one of the 8-9 piece puzzles (also presents from his Grandma Kath) that had been sitting on a bookshelf in his bedroom for month unnoticed. That was that. I found the others -- I'd put them away months ago -- and suddenly he was solving them in a flash and yelling for more.
Melissa and Doug, who made the four we had at first, seem annoyingly to be have added sounds or music to their classic 9-piece puzzles (like the one above), when one of the best things about puzzles as far as I'm concerned is that they're quiet! So we hit Target and found an armload of Circo puzzles instead.
Cars, Trucks, and Buses
Since his colossal interest in all things vehicular first manifested a few months ago, he's built up quite a collection in various materials and scales, from Matchbox cars to a wooden dump truck that was mine as a toddler and probably weighs ten pounds. He has the fabulous little Plan buses with smooth-running rubber wheels at left, and he has a seriously irritating Fisher Price schoolbus that makes a dozen different noises that all make you want to kill yourself. (And, wow, did Little People ever get super-lame.)
It's hard to say which among these are his favorites. He has affairs with certain ones for a few days and then casts them aside for others. He does in fact take them to bed -- the smaller ones, anyway. At bedtime and naptime, he picks one or two to take upstairs. He doesn't cuddle them, but they do appear to be essential to the going-to-sleep process somehow.
My favorites, of course, are the ones that don't make noise and don't have pieces that come off and need to be incessantly replaced.