With static subjects, I use One Shot. With my 5DII, I almost always use One Shot because of the disappointing results I get with AI Servo, regardless of lens. On my 7D, if there's movement or expected movement, I use AI Servo. Shooting with my 85mm f/1.2L II on the 7D in AI Servo, several times with young daughter running diagonally toward me, I got about 8 of 10 shots in crisp, eyelash-counting focus despite the 'ponderous' AF of the 85L..
what focus mode ddo you use? zone? spot? i havent had such great results using spot and trying to maintain tracking of a persons eye walking towards me
Depending on the subject, either AF point expansion (the + pattern with 4 supplemental points on the 7D) for subjects I can track pretty well in the VF, or Auto selection (where in AI Servo, you designate the starting point for tracking and it then uses all 19 as needed to keep a lock) for very erratically moving subjects.
Spot AF doesn't work well for moving subjects (IIRC, one of Canon's technical write-ups advises against it, as well).
I don't understand why people always claim that on the 5D3 the metering does not follow the AF point achieving focus or pre-selected to achieve focus. Of course the camera meters at this AF point as long as you use it in evaluative mode. It's at the bottom of page 171 of the manual and I can confirm that.
Let's be clear, though. Evaluative metering is linked to the AF point selected (by you or the camera), but evaluative metering is not the same as spot metering. Evaluative metering looks at all zones, and compares the luminance (and for some metering sensors, two-color or RGB color values) readings to a stored database of pre-programmed scenarios. The final meter reading is weighted toward the AF point (degree of weighting is unknown and almost certainly variable), but the final reading comes from the whole scene. That means a large bright or dark portion of the scene, or a subject that has a very different luminance that the rest of the scene can throw off the metering for the subject. Spot metering ignores the rest of the scene, and when linked to the AF point as it is on 1-series bodies, is very useful.
The classic example is a bird flying across a blue sky - evaluative metering, despite a 'linked' AF point right on the subject, will give you a silhouette every time.