I don't/can't really do favourites but if I had to pick a male and a female whom I've never seen put a foot wrong and completely disappear in to their characters I'd probably go for John Hurt and Jessica Chastain at the minute. Hurt in particular had an almost magical quality to bring life to otherwise dull roles. There's lots of actors who've had exceedingly good singular roles, or transcendent roles under certain directors, but I'm trying to rate on overall career performances.
Richard Burton would have been in the running for most of his output (Who's Afraid of Virginia Woolf? probably being one of my favourite films of all time), but I've seen a couple of films where he was blatantly phoning it in; Michael Caine similarly. Bob 'Oskins, Emily Blunt, Florence Pugh, Robert Pattinson, Sam Neill, Jake Gyllenhall and Olivia Colman are all exceedingly strong contenders. Al Pacino would be in there too if I could ignore the excessively hammy aspects of his acting that seem to have been demanded by most directors post-Scarface.
Reading it back I seem to have a strong bias for brits but I'm hoping that's just because I prefer the "classical" theatre style.