TRS-80 Color: Strangely, I had an old beat-up miscellaneous cartridge for it (Galactic Attack) for years before I ever even got a system to play it on. My first impression was that it was a little strange in some ways, but once I started getting more games for it and learning about its fairly huge third-party catalog of tape/disk software, it quickly became a favorite.
VIC-20: For many years I didn't give the VIC-20 much thought, if any. I regarded it as essentially a much more primitive iteration of the Commodore 64, which I already had, so why bother? Well, I found a cherry CIB system and a half-dozen games at a Vintage Computer Fest a few years back for the $50 that was burning a hole in my pocket. I was delighted to find that everything I thought I knew about it was wrong. Fun little thing. And VIC-20 carts are some of my favorite things to collect now.
Apple IIGS: First of all, I had never expressly set out to collect Apple stuff at all. At least as far as my hardware goes, almost all of it is just stuff that I lucked into (except my II Plus...that thing, I wanted). //e systems, printers, joysticks, monitors, all of it. Over the years I wound up with quite a pile of //e and //c parts, peripherals, drives, etc. That was good enough for me. I knew about the IIGS, and that it wasn't really as good a 16-bit system as stuff like the Amiga or Atari ST (which I had). A few years ago a buddy of mine messages me asking if I wanted one--I hadn't been looking for one, but the price (free) was right. ROM3 with a memory expansion and RGB monitor. All I had for it was a 5.25" drive so it was essentially just a snazzier //e until earlier this year when I finally got a CFFA3000, after which I've revised my opinion about IIGS a bit.
ADAM: I'm still pretty new to the ADAM, so I can't really comment too much on it at this point, but I'll say this--it's such a weird system. Not necessarily in good ways, but at least in interesting ways. My system came from Milli, with an internal power mod, so I don't need the printer, but that power-thru-printer design is one of the most immediately oddball things about this system. The other is the Digital Data Pack drive. I noticed it has none of the FF/RW/Rec/Play/etc buttons universally found on tape units. That's because ADAM tape drives actually act like disk drives--the system automatically peels through the tape (at noisily and terrifyingly high speed) looking for files and data in much the same way a disk drive seeks on a floppy. At first I was a little perplexed that ADAM CP/M came on tape--"How the hell can you run a disk operating system from tape?" Well...the ADAM DDP, that's how--the tape that thinks it's a disk. It's faster than a standard cassette interface, but also far slower than an actual disk system. But it's definitely interesting!