Basically... the entire Classics (and associated) and Animated lines... Didn't start with Animated until the show was already over... Needless to say, I barely managed to snag any of the figures. Blurr, Sentinel, and Ratchet were all that was left at my local Walmart, later found a majorly marked-down Leader-class Megatron (bought it in a heartbeat), later found Optimus Prime, Grimlock, and Blitzwing at a Toys 'R Us (also seriously marked down), and found Snarl, Bumblebee, and Oil Slick at that same TRU for full price. I had to order Arcee and Shockwave.

But with Classics, I only recently started collecting that type of mold with Generations.

 

ROTF Prime.. I should've got him

 

I wish I coulda gotten a Buster Optimus Prime and Jetfire 2 pack

 

I once passed on both a Universe 2.0 Smokescreen and a Hound/Ravage for Cyclonus. Never saw Smokey or Hound again. Can't say I regret it because I love Cyclonus, but I do wish I had another shot at Hound/Ravage.

Also, most of the Armada line. For the series that really got me into TF's in the first place, I have a scant few larger figures even though most of them were always on the pegs during my childhood years (Mini-cons are a different story, as I have a crap-ton of them, although ironically I'm still missing my favorite; Leader-1)

 

Shadow Scyther and the "Snowman" Superion upgrade, both were in my online carts at some point and I changed my mind.

Most recently, when I was at Botcon someone had a ~60-70% complete Metrotitan, loose, for $200. Chose not to get it. Walked by it AGAIN the next day and thought about it harder. Chose, again, not to get it.

Changed my mind a few hours later, walked by, gone. D'oh! That's what I get.

 

Fansproject City Commander, ROTF Bruticus and MP Thundercracker!

 

G1
Fortress Maximus
Trypticon
The Predacons

BW
Tripredicus
Mangaboss

 

Cybertron Excellion (I think)
ROTF Brawn
ROTF Lockdown
ROTF Dune Runner
HFTD Sidearm Sideswipe
HFTD Devastator

 

Energon Shockblast
Cybertron Vector Prime (though I bought the Universe repaint eventually)
Classics Grimlock
Animated Grimlock (seen him a lot of times but didn't buy him)
Human Alliance Mudflap (only seen him once and I didn't buy him, BIG mistake on my part)

 

Warbot Defender. Most definitely Warbot Defender.

 

City Commander. With out a doubt.

 

Seen like 3 Windchargers at a WalMart and passed on them (Im a DUMBASS!!!!)

 

Energon Omega Supreme for £15 MIB in a second-hand store.

Maaaaaybe Classics Onslaught.

Honestly, there are far more I regret buying than missing.

 

CLassics Skywarp, Mirage and Cliffjumper

 

I didn't know US WalMarts even got Wincharger...

 

City Commander
US MP Starscream
Most of the Classics Line