For me: Most of the TR line. I liked the headmasters, but making almost everything a headmaster kind of wore me out.
I know that feel. 4 years later and I'm still kicking myself for not snagging an Arms Micron Breakdown on eBay for £40 when I had the chance. Still being new to collecting at the time it seemed a silly amount of money to spend on a toy, but if I could find one now for that price I'd hit "Buy It Now" faster than the speed of light.
Anything that I bought just to scratch that collector's itch. In this case, that would be most of the TF: Prime and Beast Hunters mainlines, many of the AOE toys (fucking Slash, man), and almost all of the FoC figures. I've been doing this too long not to know my own tastes and I knew going in that I didn't really want most of those toys. But the itch. It got the better of me.
My all time worst would have to be a tie between 2001 Storm Jet or the 2010 Darkmount. Both are complete garbage.
I regret purchasing the AoE figures that later got a premium or "platinum" redeco in a box set. So pretty much all of them. That is why I am waiting until after I see TLK and SDCC to start buying any figure from TLK, except leader Megatron. I also regret TR Voyager Prime and Megs simply because Prime came in the Chaos on Velocitron set and I think Megs will be in a future box set in a more G2 deco.
Yep, no purchase regret is bigger than any non-purchase regret. And any purchase regret is easily rectified via the online marketplace, given time. I regret buying the CW Computron (probably my biggest regret buy), but after some time I could sell him with no loss at all since his value grew. There's always somebody out there who will value what you don't. Not buying the reissue of G1 Powermaster Optimus Prime? With time his price went up and the regret only grew more and more.
The only one that I have had recently is Titans Return Skullsmasher (Skullcruncher). It was a nostalgia-influenced purchase, and I kinda hate the figure. Now, I have him in gator form, just sitting on my desk, and he'll likely sit there like that for a long time.
A custom dotm megatron made from Rotf megs. Fragile a s$&@, cost a crap load and now I can't find a buyer to get rid of it Worst decision ever
Have had many regrets buying TFs over the years. Here are some of mine: *Most of the AoE line. I loved the simpler designs and I pretty much bought every Generations release except for Hot Shot. Few months later, had buyers remorse and decided to sell them all except for the Dinobots. *MP Star Saber. Dont get me wrong, I thought he was a pretty decent figure. But the soft ratchet joints in his arms were a letdown and the screws were rounded out straight from the factory so I couldnt fix them. Just overall, felt cheap. More like a $50 toy instead of a $150 MP. *Hasbro MP Sunstorm. Got him home and he was a floppy mess. One wing wouldnt stay up in robot mode, his torso kept sliding up when I would pick him up. Dont live near a TRU (closest is 80 miles away)so I couldnt just return it and by the time I was near one again, the return policy expired. Ended up selling him at a big loss. *FoC Bruticus. The combined mode was allright but the individual figures felt like huge steps backwards in design. Thankfully CW was a little better pulling off the combiner gimmick. *Henkei Thrust/Dirge. Bought these guys for $150 at BBTS I think only for Hasbro to re-release them at regular retail.
This right here. Those damn impulse purchases! I would add a few of the NuRID figures to that list as well. As far as regretted purchases that weren't impulse... Masterpiece Tracks and Road Rage. I bought Tracks hoping that all the negativity I had read was just from overly nitpicky collectors. I was wrong. Transformed him twice and put him on the shelf. When I was able to find an official Road Rage discounted to KO prices, I bought her too. Opened her up and all the reasons why I dislike Tracks came immediately rushing back. *sigh*
The armada line, the autobots in the energon line, MP Prime, MP Starscream, the alternators line, every combiner fansprojects has done, FoC Bruticus, FoC Bruticus upgrade sets. I don't regret the movie figures I own, some of them are a lot of fun but the main on screen cast never get displayed so they could go on this list I guess.
Very few as most of my purchases have been 10.5 years of mostly Classics with odds and ends like Tit, ROTF,MP,Cybertron, RID2001. 1) 2008 Universe Galvatron. Floppy mess, looks like crap in robot mode, arm kept popping off. Ironically, the headsculpt and light piping were awesome on a crappy body and now TR Galvatron has a great body/tf sequence with a crappy head. The universe works in strange ways. 2) Energon slugslinger.