The Megaforce Zord Builder system.

Two years ago I would have said the OOO Toyline but then I went to japan and pretty much got all of it.

And thats pretty much my stance now on Lines I have missed. I try to go to japan every few years so whatever I have missed out on I can buy there.

Gekiranger's.
I hadn't known of it's greatness, and I just skimmed over the Jungle Fury line (Even when I saw the zord pack that included RinLion and RinChameleon! *Cries*)

Well I really have nothing I regret skipping on, but I wish I never sold off my Jungle Fury figures. I assumed Bandai was going to do Figuarts of them when they revealed Rio and Mele prototypes, so I got rid of everything, the five rangers, Rio, and Mele. I also wish that I had bought the mech back of Wolf, Black Lion, Chameleon, and then the rhino.

My only regret is returning the 2010 Megazord. Considering how much the Legacy version was/is now, I had a cheaper version for only 20 fricken dollars and I returned it like an idiot. I don't even know why I did it.

Legacy Megazord line. Saw Dragonzord and was tempted, but without the original Megazord to go with it, didn't feel like I could justify it. And we'll be having the fourth entry in it soon, so it feels like I hesitated just too long to have a hold on collecting them.

And I do wish I got into Figuarts sooner than I did, since I knew of them well before I bought any.

I regret everything. The only line I have complete are the Legacy line, the Super Megaforce line, and the Dino Charge line (so far). I'll soon have the Dairanger line complete but I need to replace my red dragon thunderzord and I still need Serpentera. Still my tor needs repairs and I need the red min-figure and it needs new decals, but is almost finished. And these are just the megazords. I only get megazords. I rarely ever get figures unless they are necessary or for nostalgic reasons. Or it looks really cool. Oh and I have my Jetman collection almost complete. I am just missing Tetra boy.

Probably Jungle Fury. I once had the opportunity to purchase the giant giftset that included the smaller version of the Jungle Pride Megazord and the six auxiliary zords(plus I think a leg zord that was remolded to look like their cannon?), but passed on it since I already had the DX megazord. I really liked those zords too >_<

Kyoryuger :(

I personally wish I got some of the Kyoryuger stuff as well. The mecha line for Kyoryuger was pretty pricy even on sites like CStoys since the boxes for them were pretty big which boost up the shipping for them.