I would keep all of them. I purged my collection once and I've since come to regret it so I will never make that mistake again. I'm holding onto my collection until the die I die, and if I lose interest in it, it'll go into storage, not onto e-bay.
G1 Grimlock, I have some PRICEY TF and some very dear to me. but it took me almost 20 years to get my hands on him. was one of the very first things I did when I started working.
For me the one figure nothing could make me part with is my MP-08 Grimlock from Takara,to me he is the Crown Jewel of my collection. But if I could keep more than one they would be my G1 Mirage (he was my first TF,and still have him to this day),Fortress Maximus even tho I have no accessories for him just the body,and my nearly complete Classics Seeker Set (I have 1.0 Screamer,2.0 Screamer,Target Skywarp,Acid Storm,Ramjet,Thrust,and Dirge)only need Thundercracker coming in a month or two and that be all she wrote. Oh hell one last one ROTF Voyager Mindwipe HOLY SH** he is the best designed movie figure ever,no alt mode kibble underneath,sick a** bat mode hanging upside down,and his bot form is awesome as crap too.
Everything is potentially disposable. But parting with what masterpieces I have would be very very difficult.
The same is true for me, but sadly I lost mine loooooong ago. Along with green Beachcomber, gold Bumblebee, and shiny red Hubcap. Whew, I have a few. Faith Leader will not go anywhere, he was a combined gift from my girlfriend and my best friend. Animated Shadow Blade Megatron was also a gift from my girlfriend. I also just pulled out my old, horribly beat-up G2 Prime, and I'm sprucing him up with BTS-04 and Reprolabels. He still won't be worth much (his trailer is a MESS, no Roller, etc) but he's my first Prime and spending a few extra bucks will make him look cool in a whole new way. Universe Dinobot was the first TF toy I bought for myself in college, starting my modern collection. He stays. I'm also sure I'll always have a Starscream, and always a version of the Cyclonus mold (don't know why but I love him, have two). In general I don't think my collection is worth that much (modern toys since 2008, a few add-on kits and older Classics), and hope not to be in a position where selling it makes a big difference. If I really needed a shot in the arm I'd sell all my FP stuff, much as that would suck.