Davros

Davros, we all know him course as the creator of the Daleks. Originally a Kaled scientist who was confined to travel machine due to war injuries , a travel machine which resembled the lower half of a Dalek. Like the Daleks, Cybermen and the Master, more on him soon, Davros was an enemy who would not die. Despite being stranded on Earth, apparently being on an exploding ship, captured by humans and also his own creation, even being exterminated by his own creation did nothing to stop this evil genius.
When the Daleks returned in 2005, many thought that Davros would also make a return, despite being stranded on Earth at the end of his last story from the Classic series, Remembrance of the Daleks. However, this proved false and it was not until 2008 that the last of the Kaled race would make a return to the show. Once again, though, it appears as if Davros is gone, supposedly choosing to remain on a space station that then proceeded to blow up.
Is he dead though? He has shown us that he is not the easiest person to get rid of, will we see him again? I deem this very likely.

Davros

A crippled Kaled scientist

The final creator of the Daleks due to Time Lord interference

Originally exterminated by his creations

His subsequent refusal to die led a Dalek civil war

A depraved and insane megalomaniac but still retained his brilliant mind

After the Time War, became more defeatist after realising the Daleks had moved beyond him