My vote goes to Picard

First of all, Picard is no stranger to the auto-destruct sequence, as in "11001001" and "Where Silence Has Lease". The crew complement of the Enterprise-D is much higher than those of the other primary Star Trek ships that we encounter, and also includes families and civilian employees. (See this answer for crew complement information regarding the Enterprise-D.)

In "Lower Decks", Picard sends Ensign Sito on a mission for which the chance of survival is known in advance to be extremely low. In fact, she does not survive.

Finally, in Star Trek: Nemesis, Picard rams the Enterprise-E nose first into the Scimitar, without ordering an evacuation of the forward saucer section (nor did he give time for one).

At best, there was:

All hands, brace for impact.

In fact, I had previously raised a question about this:

Warning regarding forward saucer section in Nemesis?

Well, according to a contribution to Cracked (entry #7) by someone calling themselves RogerRamjet:

Percentage of the Crew Killed While They Were the Captain

• Captain Jean-Luc Picard: 7%
• Captain James T. Kirk: 22%
• Captain Kathryn Janeway: 37%
• Captain Jonathan Archer: 67%

...not counting the books, comics, movies, post-death revivals, dream sequences, parallel universes, or multi-death timeloops.

Personally, I'd vote for Commodore Matt Decker, in "The Doomsday Machine" (TOS). He attempted to save his crew by beaming them all down to a planet, only to then watch helplessly as the machine then consumed the entire planet. That means he managed to kill off his ENTIRE crew in one shot (and went a bit off the rails due to the resulting grief).