Was the mitochondrion or chloroplast first?
Mitochondria evolved before chloroplasts.

We know this because Mitochondria form a monophyletic group: e.g. all life with mitochondria traces back to a single common ancestor (source). Since the group with chloroplasts groups within this clade, it must be the case that either (a) chloroplasts were obtained by an organism that already had mitochondria or (b) chloroplasts were independently lost by multiple lineages within the Eukaryotic clade and then many of these lineages re-acquired chloroplasts by secondary endosymbiosis. Since (a) is a (much) more parsimonious explanation it is the one it makes sense to accept.