I don't know if I can choose a "Top 5." I have many items, and if I didn't care about them or think they were special in some way, I wouldn't have them. I'll just kind of riff off some things that stand out to me, how I got them, and why I think they're special. Pick five, then, I guess.
Item: TRS-80 Model II (badged "Tandy 2;" the only one I've ever heard of like this) w/ Disk Expansion Unit
How obtained: I helped a certain well-known video game historian move his collection last summer, and he apparently didn't feel like moving these items (very large, very heavy, no software, no place for it), so he gave them to me as compensation for helping him move.
Why: I got it basically for the price of hanging out with a good friend all day and helping him out, things I would happily do anyway. Also, it's a pretty unusual model. There aren't really any games for it to speak of, but I'm hoping to put together some simple BASIC or CP/M conversions. (If anyone has or can point me in the direction of program listings for some Roguelikes, hit me up!)
Item: TI99/4a Peripheral Expansion Box
How obtained: In a lot of TI99/4a stuff on craigslist (system, P-Box, dozens of cartridges, other accessories), all for $10.
Why: Instead of ~$50-$100 (or whatever they go for these days), I only had to pay $10 to discover I have pretty much no use for a P-Box, other than for it to look cool in a TI setup.
Item: Stadium Mud Buggies (Intellivision)
How obtained: A freebie in a lot of 12 Intellivision games (Buy 5, get one free, x2) that also included the likes of INTV Pac-Man, Diner, Pole Position, Centipede, River Raid, and Sewer Sam...all for $10 at a local game/card shop. (Not making that up!)
Why: Do I really need to explain?
Item: Apple IIe + Color Monitor + DuoDisk unit
How obtained: Goodwill. All for $2.99.
Why: Damn good deal. The IIe turned out to have a cooked motherboard (D'oh!) and the DuoDisk is a deskweight (D'oh! D'oh!), leaving me with a perfectly good Apple Color Monitor (Woohoo!) for $3. I'd make that deal all week.
Item: CIB Aquarius computer with boxed Mini-Expander, CIB AD&D: Treasure of Tarmin, CIB Tron: Deadly Discs, CIB Snafu, and boxed Astrosmash.
How obtained: Craigslist, $30. Also scored Miner 2049er (Colecovision) in that same haul.
Why: Another damn good deal, I'd say. At the going rate, the Aquarius wasn't a system I'd have gone out of my way to get, but I'm glad I was able to get this weird cousin of the Intellivision. I look forward to snagging an Aquaricart one of these days!
Item: Boxed Fairchild/Zircon Channel F System II
How obtained: eBay.
Why: It WORKS. I've owned four or five different Channel F systems, and this is the only one that works 100%.
Item: Channel F Dustcloth
How obtained: ebay, with a defective "untested" Fairchild VES.
Why: I don't know of anyone else who has this. I'm sure there's got to be someone out there, but I don't know about it.
Item: Boxed Sylvania Intellivision
How obtained: Bought it with 12-15 CIB games from a friend of my brothers at the time for $40.
Why: It was my second-ever "retro" system (after the Odyssey 2) I got when I started collecting. Didn't know it was rare, and for years it was my only Intellivision, and was used extensively; it has since been retired.
Item: Anti-Aircraft arcade game (Atari, 1975)
How obtained: Bought at MGC 2010 for $175
Why: My first arcade machine. The previous owner added a toggle switch on the board for airplanes or UFOs, too.
Item: Odyssey 2 system w/ a dozen or so games
How obtained: Traded a friend a handful of Playstation games for it in the late '90s
Why: It was the first system I got when I started collecting. I have fond memories of going to my friend's house after school to play it (even at the expense of the Playstation), before deciding I had to have it. Luckily he actually had two (one came from an aunt who had no use for it), so it didn't take much arm-twisting.
Item: My first copy of Star Ship (Atari 2600)
How obtained: A nearby game store, for a buck or two.
Why: I only knew of it vaguely, but I became smitten by it. It got played for hours that night by me, my sister, and my now-fiance. Good times. Star Ship is still one of my all-time favorite 2600 games.
Item: Boxed Atari Super Pong Pro-Am Ten
How obtained: eBay, for just under $50.
Why: Nostalgia, I guess. I remembered seeing one of these forgotten and covered in dust at my aunt and uncle's cottage about 15 years ago, in my pre-collecting days. I was fascinated by it.
Item: CIB Odyssey 500
How obtained: eBay, for $15
Why: Part nostalgia, part cool factor; I remember reading about it in an old EGM article about retrogaming, and then wanting one. And, having actual graphics instead of just blocks for paddles (in color, no less!), Odyssey 500 stands out in the ocean of mostly-identical pong systems.