I vote for Blake. Here's why... Zachary has an -ry ending, which is too reminiscent of the middle name. Jenson has two syllables, the same number as the middle name, which gives the name a staccato feel. Rhys begins with the same sound Henry ends with, again giving the name a repetitve feel. Blake is a single syllable with no sounds similar to the middle name. It'd sound especially good with a three-syllable or single syllable surname, if you happen to have one. :)
(I'm not a baby naming authority or anything. I've just read a lot of poetry, so I have a lot of weird ideas about what sounds good together.)