As a technical illustrator, I tend towards my annotations fonts being sans-serif, simple, and clean, with high legibility even at smaller scales, and minimal or no decoration. I have, on occasion, when I know my final target output is only print, used a simple, non-slab serifed font, but that's pretty rare for me.
Although I've a fair few issues with a number of the conventions of mechanical and architectural draughting (where I've spent a lot of time) and the lettering thereof (Seriously - all caps? Really folks legibility, clarity and beauty matter, you know!) as practised here in the US, they have typically preferred simple, clear linear hands and fonts - originally for ease of hand-lettering, and later both by convention and for ease of legibility, and one can learn from this that when the focus should be the illustrative content to which the annotation is hierarchically subordinate (but supportive) then one should seek hands/typefaces/fonts which are highly legible, simple and clear, and which eschew focal attention or limelight - the textual content is key, the design of the letterforms in this case should basically disappear. This doesn't imply that you cannot use a well developed typeface (I've used Helvetica for annotations before, and it works quite nicely) but rather that in this use-case, the typography is subservient to the extraction of meaning, rather than the design being predominant.
See several examples:
Hope this helps a little.