The two I regret most are SA 1976 and the XDA Scrabble set. I’ve been around for a good few years and I’ve not found too many must haves but these sets are two that twang that regret string whenever I see them.
I’d be interested to see if it’s just me, or if there are any other people out there with a similar predicament.
XDA Oblique for sure. A bit before my intro into the community, but what a lovely set.
So many. There are lots of GBs that I was interested in that I just didn’t have the disposable cash for at the time.
Couple that come to mind:
GMK Taro - This was the first keycap set that I was seriously thinking about getting. However I choked with the reason of “130$ for plastic caps!?”. I just was not in the right mind set for this hobby at the time.
I almost got in on round 2 as well, but choked again because it was/is so similar to GMK Mizu which was on the way. One of these days, I may pick it up or see if someone will trade Taro for Mizu.
Polaris and Vega - At the time, these seemed like such no brainer boards to grab, and for the Polaris, I had the cash. I just was not really into another 60% at the time. Now I have a KBD8x and I love the design language of ai03’s Reference series. The after market prices are probably worth it, but I have a personal goal to never pay aftermarket prices for any board.
Oh another one: ePBT Extended 2048! I have a feeling that this will come back eventually. There was a second round but it was for Topre style caps. 
Oh yes. That’s a classic. I tried XDA blanks a few years back and hated them. Swore off them. Got a new set (with legends) and now I love the profile. Funny how taste change.
GMK First Love. I’m a big fan of the black and purple design, but I was still quite new to the custom scene when it was live (I didn’t even know what a Group Buy was).
Oh yes. I’d not even heard of that one. Always nice when there’s a good story behind them. Those novelties too…
There’s a handful, but GMK Phantom is the big one. Don’t even know what I’d put it on - it just looks amazing.
Metropolis. I had a set I got off mm from R1, but paid too much to get it so I had to resell it at the same price to get my money back and then when R2 came up I don’t know why I didn’t get in on it.
Masterpiece :’(
DSA Milkshake, GMK Muted r1, GMK Monkey, SA Senna and probably more that I can’t think of
SA/DSA Retro, Lunar AEK come to top of mind.
I also generally regret not going on a buying spree in 2018-2019 before the hobby exploded and prices skyrocketed.
Time80 Reforged - I really like the design but I was so picky about the colors. At the time, I was looking for a keyboard with a specific teal/blue color that would fit perfectly with GMK Yuru. After joining CRP R4, I wish I went for the silver Time80 to go with CRP Tulip.
GMK Burgundy - I didn’t have alot of money back then, so if R4 ever happens (hopefully with a Canadian vendor), that’s my only chance. I really like how it looks with black, red or polycarbonate board.
7V - The only 75% board I like but didn’t have enough money. I don’t mind waiting for R2, which I know it’ll take a while since Gok is busy with Venn and Type K.
7V. I went with Rukia instead. I’m still sore about that.
GMK Alter. Saw talk on Switchmod’s Discord that it won’t be offered again due to GMK changing terms due to copyright/IPs (or something like that). Also, GMK Future Funk and Analog Dreams. Though, I’m happy to see that Analog Dreams is currently having a round 2 and Future Funk will have extras offered in the near future.
None anymore tbh, I’m over it. There will always be something else to get and there is so much stuff all the time anymore it’s not worth stressing about stuff you don’t get
Oh, also I super regret not getting in on the Omega 60:
That was a huge miss on my part, also not helping to nudge Windeh’s other 2 projects into GB stage.
Feels like I had the opportunity to get SA 1976 multiple times during 2020-21 from both vendors and after market.
But my biggest regret is:
GMK Phosphorous
And then there is a bunch of things I want now but I discarded when the GBs were live, like:
GMK Future Funk
GMK Nautilus Nightmares
GMK Cooper
GMK Firefly
There were several sets that looked good in renders but I didn’t buy at the time:
-Mizu
-Shoko
Fortunately, I was able to get in on Shoko R2. Which will probably use better molds than R1.
The one I feel a ‘twang of regret’ for is JWK Classic Cyrillic Tripleshot, which looked awesome and turned out awesome, but I wasn’t going for single-colour non-Dolch sets at the time. [Also, I could have gotten in on ePBT Slate].
MT3 Godspeed. At the time I didn’t have my first MT3 set and then I later got serika and it is by far my favorite profile (with KAT being in second)
v4n r1
i was right there just didn’t pull the tirgger!