heads-down, regular freestyle is going to be your best option. You said you're proficient in freestyle. So if you're asking about heads-up freestyle- is there a reason you can't put your head in the water and do bi-lateral (or single sided) breathing?
You'll naturally lift your head to the front when sighting (every 3-5 strokes or whatever is comfortable for you),but a regular freestyle stroke is your best and most efficient one.

I've seen people do breaststroke in a tri, but having been kicked by someone twice doing the breaststroke, I think it's not an appropriate stroke to do when people are around (one time it knocked the breath out of me, the other time it kicked my goggles off and left me with a black eye- those kicks are powerful). I guess if you did the breaststroke to the side and nobody is around it would be OK.