I know many people have certain characters that without these characters, the collection couldn't be complete. What characters for you are required to have a complete collection?
I mostly collect CHUG, so in order for me to have a complete CHUG collection I want at least an update to every Autobot and Decepticon character released between 1984 and 1989 from the Autobot Cars/Seekers to the Pretenders and I'd say I can stop collecting cause my number one collection is completed. Heck while they are not necessary I am throwing in some cartoon only and comic book only characters like Elita One, Devcon, or Impactor, Japanese only characters like Minerva, G2 characters like Dreadwing & Electro, heck even throw in Scourge from RID or Slipstream & Lugnut introduced in Animated but got CHUG figures while I'm still waiting for Hasbro to get to 1989 in terms of characters. Edit: My animated, cinematic universe, and aligned shelves meanwhile don't really have criteria character wise that I need to complete said collection cause I'm not really focused on them like my CHUG collection so I could careless if they are completed like my CHUG collection.
To have a complete collection ? Huh, all of them ? To have a collection with all the characters that are important to you ? Toys of those characters.
Pretty much this. An argument can be made for Fortress/Grand Maximus as well, being the biggest TF ever before the Generations titans showed up. Tough question, considering the "what" of a complete collection can vary so much from person to person, from one example from each of a few favorite lines to "every variant of every everything".
By definition to have a complete collection you need 'em all! I think your question is a bit too vague. Do you mean G1, BW, etc?
If I ever get all the g1 characters I find interesting into my CHUG collection (getting very close to being finished with all the new Headmasters), I'll probably just pick up other figures to add to the collection as an "expanded universe" type thing. I already have several Unicron Trilogy and movie figures with my CHUG guys if the aesthetics match up well enough. I don't really care about the cartoon or comics, I just buy what looks cool.
Unless you have G1 POWERMASTER Optimus Prime. (Because why should Optimus Prime be a mere repaint of Ultra Magnus sans armour? ) Also:
For me to stop collecting TFs entirely? I would want to finish my WFC / FoC collection 100% (I'm not missing many for this goal, actually - mostly a few Japanese ones and some exclusives), get a few more toys for my screen-accurate BW set, and from there... I guess polishing off Masterpiece, which for me is G1 S1 + select S2 & Movie, and whatever strikes my fancy from non-G1 MP (such as BW, and probably Armada).
Hmm. That are on my wishlist? Sideswipe Jazz Thundercracker Octane Astrotrain Sun Storm Nemesis Prime Night Bird Quickshadow Wild Rider Prowl Barricade That I own? Skywarp Acid Storm Soundwave Thunderhoof Starscream Mixmaster Optimus Prime Windblade Sunstreaker That I'll never get probably? Moonracer the other rainmakers Devestator Brainstorm Sludge Swoop General Glowstrike Filch Terrashock Ruination
Ultimately it depends on what media you have nostalgic feeling towards. For me it's any character that originated from the Beast Era.
I have never seen anyone else create so many threads so quickly about what figures he "should" buy or have. Figure out what you like and make a plan. Don't collect what other people like and don't get too ambitious in the beginning. Start slow or you'll have money regrets later. For me personally, my focus centers on Sunbow season 4 and Masterforce.
For me to stop collecting I'd need the main/ supporting Animated cast as well as Wreck Gar and Rodimus, CHUG Lost Light crew plus Marvel and IDW Wreckers + CHUG favourites from G1 that aren't already covered and more prominent or favourite G1 characters as Masterpieces. Probably about halfway on those goals, more than half on Animated, less than half on Wreckers. Plus MP Xaaron is never happening, ensuring my collection remains incomplete so I keep filling that Marvel UK shaped hole with guys I don't need, hey there Universe Countdown. The true problem is focusing my purchases, do I need multiple movie Bumblebees? Hell no. Do I own them, yeah. I bought CW Optimus knowing he wouldn't become the definitive CHUG Prime and I could of used that money on Motormaster. I will probably buy Triggerhappy knowing fully well I'd pick up a crappier one that actually had Targetmasters. Is this what an addiction is like. In short: I have a problem, but hypothetically just the favourites +Animated Blackout.
For ME to finally be complete I want to get anything that was from Diaclone or Microchange in Masterpiece scale (also the '86 movie characters seem to have snuck into this part of the list...). This part is actually almost done...with many figures already announced by 3rd party companies, just Skids, Powerdashers, Bumper, Powerglide, and the Omnibots remain with no representation. Then for CHUG scale I want a complete set of G1 and G2 original characters (US, UK, and Japanese) minus the Micromasters and Pretenders. Don't care if they come from Hasbro or 3rd party...Hasbro has hit a surprising number of late G1 and filled a lot of holes I never expected to see, but either them or a 3rd party could make a mint off of me by concentrating on the original characters from G2 lol.