Malkovich would be awesome...too bad it's just a rumor. With the nearly flawless casting thus far, I have faith that whoever they do cast will be the right choice.
I've always thought Christopher Plummer should be Voldemort. If they need to cast younger, Gabriel Byrne should be considered (especially since the actor who played "Tom Riddle" in Chamber of Secrets looks like a young Gabriel Byrne a wee bit).
I always thought Colin Firth would make a great Voldemort. He's got the range & the look to act purely evil, and resembles the Tom Riddle character from the second film.
Edit: On second thought, I think Jeremy Irons would also be a great choice to play Voldemort! I had him pegged for the Ra's Al Ghul character in the new Batman film, before they gave the role to Ken Watanabe. Irons definately has the Aura for a sinister character like Voldemort.