Agreed with all that say the character has never been cast properly, though in the film-makers defense, Fleming didn't make it easy on them as I'm not entirely convinced he made up his mind!
Ranking the three portrayals, Savalas is my favourite, easily the most menacing of the three, most convincing as "bad guy." I know the "thug" comment is always thrown about, but as far as I'm concerned I find that less artistically offensive that Gray's Blofeld - he actually seems like a quite a pleasant chap - urbane, witty, you'd spend an afternoon drinking with him rather than worrying that he was up to something. Less threatening than Louis Jordan's Kamal Khan, someone else who looks like they could be quite entertaining to socialize with and Jerome Krabbe playing whats-his-name from TLD!
Pleasance's Blofeld is perhaps my least favourite of all the villains in the series. Maybe I can't separate the portrayal from the parody, but he comes across as neither physically threatening, or mentally a genius. Completely unconvinced that the criminal underworld would follow him, whereas Savalas is without doubt a leader.
Pleasance as Blofeld as the President in Escape from New York (at least at the start) - I could buy that. But little bald guy with a cat and safari suit and stereotypically weaselly voice? Emilio Largo would have taken over years before that!