How do I choose decent-quality passive component assortment kits?

Basically, look into the projects you want to build and determine what kind of passives they use on their bills of material. That should give you an idea of what you will want to have on hand.

You can also order sample kits from Digi-key, Mouser and others, but for basic hobbyist work you will likely need perhaps a dozen resistor values and a few capacitor values to get started. Order the ones you need as you go along, and over time you’ll have what you need for projects going forward since engineers tend use the same values for the same purposes (like 0.1uf for bypass and 10K for pull-up.)

As a start for digital work I’d have on hand:

• 18pf, 100pF, 1000pF ceramic (high-frequency, low value bypass)
• 0.1, 0.22, 1, 2.2, 4.7, 10uF ceramic (bypass)
• 47, 100, 220uF electrolytic (bulk filter)
• 10, 22, 33, 49, 100 ohm (series damping / terminator)
• 220, 330, 470 ohm (strong pull-up/pull down, LED limiter)
• 1k, 2.2k, 3.3k, 4.7k ohm (medium pull-up/pulldown)
• 10k, 47k, 100k, 220k, 470k (weak pull-up/pulldown)

I prefer Murata for caps, Yageo for resistors (but I’m less fussy about that.)

Analog circuits like op-amps will demand specific values which you should order as you need them, at higher precision and stability than digital work.

Inductors are more involved depending on how they’re used - power, RF, filtering - so best to order those as you need them based on your bills of material. Go-to popular vendors would be TDK, Toko, Coilcraft, Murata.

A selection of ferrites is useful for noise filtering. Beads for boards and clamp-on types for wires can work wonders sometimes. Murata is your huckleberry for beads, for clamp-on look at TDK, Laird, Fair-Rite.

Connectors - it’s handy to use snap-away 100 mil types for projects that don’t need special connectors.

Last thing. If you have a surplus place near you get familiar with it.

Buy from actual electronic hobby companies.

Time was, that'd have been Radio Shack and Heathkit, but now it's Sparkfun and Adafruit. And Radio Shack and Heathkit!

The difference between them and Mouser is they're actually targeting hobbyists, so they are more likely to have assortment kits for hobbyists.

Buy from Amazon proper

People think if they buy from Amazon, they're buying from Amazon.

Nope. Jeff Bezos has opened up every part of the business - servers, warehousing and the Amazon storefront. As a result, the storefront is basically eBay - flooded with that hellish flea-market "fell off a truck in Shenzhen" Alibaba junkstream.

The stuff actually from Amazon "proper" seems to adhere to the same national safety laws as the bricks and mortar stores... though those safety standards don't include usability or durability.

However, very little in the electronics space is actually from Amazon Proper.

The product page has a subtle sign of whom you're really dealing with, but it's almost like Amazon tries to hide it. Here's what it looked like two Website editions ago. Now, it's directly under the "buy" button.

Not picking on Uxcell, they just happened to come up for the search.

Buy from vetted vendors on Amazon

These would be when you identify responsible companies that make a good product, and just happen to sell on Amazon Marketplace.

They might do this because, like many businesses, they're already using Amazon Fulfillment to ship orders they take themselves... so listing the item on the Marketplace is pretty much throwing a switch.

For certain SKUs, such reputable vendors will be most of the sellers, since they're name-branded and low-demand, so not really worth counterfeiting. You can at least screen reputable vendors that way. Although once you identify one, you are better off dealing with them direct. You'll know they're legit if their package arrives in the normal manner of domestic shipping, not dropshipped from Amazon or ePacket from China.