Like a lot of folks here, I've been to a reasonable amount of tech conferences, events, trade shows, etc over my career.
When I was younger, I thought the best swag to grab from a vendor were the things that were "cool." A nifty tee shirt, a coffee mug/water bottle, or (on rare occasions) something really unusal like a custom LEGO set or a toy of some sort. I thought the worst swag was the ubiquitous pen and notepad combo.
That resulted in a closet full of mostly useless crap. One or two mugs are still useful (as is the rare baseball cap to keep the sun off me). But toys I'll never use, shirts that are just ads, etc are just a waste.
Now, I don't take any swag unless it's functional. I actually take pens, because seriously, I always need more (not in need of as many notepads, though).
The best swag? Things that are actually functional around the house. I love that a lot of vendors offer some sort of screwdriver or multitool. I always need more of these, and since I'm great at misplacing things or forgetting what pocket it's in. That cheap-looking Cisco screwdriver? Has a magnetic socket for the bits and a flashlight. Was the perfect thing for setting up some Halloween decorations that required opening a battery case with a small Phillips head driver. Doesn't have the torque of some other devices, but it was the right tool at the time.
I donated almost all of my swag shirts ages ago, ditto the toys and other branded crap. I've got all the water bottles I need. But tools? Yeah, keep 'em coming.