My rarest specials are customs include: from Fire Helix (made from scratch including the ponies, pic from OmineToyz), who sadly had difficulty keeping up (finances and work) so ended up being unable to continue.
Lovelauraland customs also would be quite special although of hers I only have 3 and 2 are missing their custom witchy brooms and mats.
Most special of non custom would be... hmm oh dear I have a shelf of 100 it so hard to decide
... which needs some realigning. I have been trying to get the accessories sets for the G1 princess set 2 (missing sunbeam), and accessories in full for many including G1 (e.g. brush grow, slumber party, dance prance, happy tails, newborn twins, brothers, rockin etc), G2 playsets, G3 and the baby twins and accessories for G2s. I often miss out on many as I don't get to go to fairs and the auctions have automated last second bot bids which are hard to beat. The accessories are often rarer and harder to find and collect than the ponies
Which is why when I saw the rollerskates melody with her glasses earlier I did a small squee
. The G1 families are also tricky as my last one to find, the meadowsweet dad with hair colours, is hard to find for less than $800USD (adding in taxes). I try to record pics of as much pony data as I can and accessories/original colors, combining from many different sites and sales records.