These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Arguably, EX is the darkhorse of the entire series, a status that extends to most of it's characters.
Skullomania. To this day, he STILL shows up on a disproportionate amount of character wishlists.
Among Skullo fans, there's also Skullo Lady from EX3. Mentioned once, and never made a playable appearance.
C. Jack. Mainly because he was Balrog - negative stereotypes - stupidity + cool. In other words, he was the original (albeit less gentlemanly) Dudley. It's died down with the years, but now that Dudley's in SSFIV, people want to see him and Dudley go at it.
Hilarious in Hindsight: Skullomania becomes this in 2009, where we got Kamen Rider Skull, complete with a scarf and the letter S on his forehead. Personality-wise, though, he's pretty much Skullo's opposite.
The Cycloids become a bit more amusing when you compare them to Seth (also making his debut in IV), who does the exact same moveset mimicking as them and (due to his appearance) somewhat resembles Cycloid-γ. Heck, he's even been dubbed "Cycloid Seth" on several occasions.