I wouldn’t recommend starting with Matt Smith or Capaldi. Moffat tried really hard to appeal to long-time fans that most episodes are completely incomprehensible to anyone unfamiliar with the show. The whole extended family was over and we ended up watching that year’s Christmas Special… no one knew what the hell was going on.
I would say starting with the revived show at the beginning, with “Rose”, is your best bet. However, I strongly recommend checking out the original show at some point.
Do you mean Matt Smith’s first Christmas special? What outside knowledge was required for that one? The companions have their own backstory, but they’re basically written out of most of the episode (and all you need to know is that they’re on a honeymoon, which you’re told). It’s actually the episode that got some of my family watching the show, and I think of it as one of the most standalone ones there is.
And there would be even less chance of confusion coming off of season 5. I’m struggling to think about how season 5, itself, could be incomprehensible to a newcomer. You miss River’s first story, but she’s reintroduced. Other than that, not much attention is paid to RTD’s storylines.