I have to base mine on the collecting experience as per the title of the thread, and not a commentary on my views of the library itself. What I think of the system or library, really is a separate tier list all together.
S Tier - During the 7th gen, the games were so damn abundant and cheap, you could literally find them for sale in a drug store. Everybody was selling the games then, and you could get them used for really cheap, sometimes brand new on clearance prices cheaper than used. It was a buyer's market.
A Tier - These were your common video game pickups, often times could get good deals on them, nothing too hard to find at the time. Retro platforms in this tier were prime in my area for the picking and were not valuated high in the least when I first started. A $2 SNES game was not uncommon. I once picked up a SNES console with DKC 2 & 3 included for a whopping $5.
B Tier - These were the consoles that seemed like they should have been easy to collect for, especially in the early days, but they were a little bit tricky to find or if you did find them, the selection was always the same stuff again and again nothing you were specifically looking out for. Occasionally I'd catch a break and find some better sources for these, but not often. Prices weren't terrible, but finding them was the tricky part.
C Tier - These were pretty hard to collect for, but not impossible. I feel like I mostly got lucky when I did come away with any scores on these, some of the stuff you stand very little chance to find. When you'd find a source with lots of other inventory, they'd rarely have anything notable for these. Had to be persistent in poking around or traveling to find stuff, and just go online half the time because you couldn't find it.
D Tier - Basically impossible to find locally, or anywhere within reasonable distance. For the most part, had to go online or to gaming expos to find people selling this stuff, and then pay through the roof to get it. Collecting for these has been a long and painful process. In fact, I gave up some time ago on collecting Game Gear or Master System. Literally everything would have to come from eBay in addition to everything else I'm always hunting for, it just becomes too excessive. For the other consoles in this tier, I tended to get maybe 2 to 5 games for each per year? It was tough.
I do own a few other platform, but I don't consider myself a collector of those, so not shown on the tier list.