What's the best ND filter to buy to get the most shallow depth of field while taking pictures?

Question

I have a Rebel SL1 by the way.  

Answer 1

This depends on if you're trying to use flash at the same time.
Outdoors in full mid-day sun, the exposure based on a "Sunny 16" rule is f/16 and the shutter speed set to the inverse of the ISO speed. At ISO 100 you'd use 1/100th. But if you dial the aperture open 5 stops to f/2.8 then you can dial the shutter faster by 5 stops to 1/3200 and have an equivalent exposure.
f/2.8 is fairly shallow, but it is not the shallowest... f/2 and f/1.4 are even more shallow. Canon makes 50mm and 85mm f/1.2L (L series lenses) which can create especially shallow depth of field (particularly the 85mm). Canon used to have a 50mm f/1.0 lens but they have not made that in years.
Depending on your subject... there's a point where the depth of field is so shallow that it's not possible to to get your entire subject into acceptable focus. At f/1.4 and f/1.2 the depth of field can get REALLY thin (especially at close focusing distances.)
While most of my lenses can handle f/2.8 or better, I often find I prefer f/4 unless I can control subject placement well enough that f/2.8 (or lower) can still work.
Your SL1 caps out at 1/4000th sec shutter speed. So if you were to shoot in full mid-day sun (a bit of an extreme example) and you wanted to use, say, an f/2 aperture, you could not halve the shutter speed to 1/6400th because your shutter doesn't offer that setting. But you COULD do that with just a single stop ND filter (ND 0.3 -- when represented this way each "0.1" worth of density is equal to 1/3rd of a stop of light. So 0.3 is 3/3rds or 1 full stop.).
I have a 2 stop (ND 0.6), 3 stop (ND 0.9) and 10 stop (ND 3.0) filters in my bag.
If you use flash, this changes things... with flash (which I usually use during the day to fill the deep shadows and soften the light) you are capped at the flash sync speed. For your SL1 that's 1/200th sec (or slower) unless you have a speedlite that supports "high speed sync" mode.
In this situation, you might actually want 4 or 5 stops worth of ND if you are shooting in mid-day sun using fill-flash that doesn't support high-speed sync. You can change the shutter from 1/100th to 1/200th (1 stop) but you'd still have 4 more stops to make up to get from f/16 down to f/2.8... one more stop to get to f/2.
You can "stack" filters, but stacking increases the odds of artifacts from reflections... use high quality anti-reflective coated filters (this is a situation where it doesn't pay to buy budget filters.)
The major caution is that there is a point where the shallow depth of field is "too shallow" for the subject.
Tim Campbell
5D II, 5D III, 60Da