This you posted this in the Rebel forum I'm going to assume you have an APS-C body and probably also have a "kit" lens (an EF-S 18-55mm f/3.5-5.6).
The "kit" lens is probably about as wide as I'd prefer to shoot for a large group. I have lenses that go much wider but there are some nuances of wide-angle effects that make them great for landscapes but less desireable for group photos.
As for "best" lens... there are some amazing lenses out there and you can spend thousands of dollars on a lens. But I suspect this is not really what you're asking. I think you are probably trying to understand what focal length should be needed for a group shot of some size in order to fit them all in the frame.
For that, I refer you to this link with photography calculators: http://www.tawbaware.com/maxlyons/calc.htm
Scroll down to the section labeled "dimensional field of view" calculator section. You'll need to punch in a few values.
You'll need to test some focal length guesses to and check the calculator's answers to decide which focal length will fit your needs. YOU type in the values they label in RED and the calculator provides the answers in the fields labeled in BLUE.
For example... let's just punch in 20mm as a starting guess. Enter that (20) in the box labeled "Lens focal length (mm)".
Next, enter your camera's sensor crop-factor. This is a fixed value which never changes (for your camera). If you own any Canon camera that has a Canon "APS-C" size sensor (every Canon Rebel camera model has an APS-C size sensor) then your crop factor is 1.6. So you'll enter 1.6 in that box.
Lastly, take a wild guess as to how far away you think you'll be standing from your group of people. Let's just pick 15' away and enter that value.
Now click "compute" and read off the answers to the right.
The answers tell us that using 20mm and a crowd of people ... all of whom are lined up at least 15 away from you... then the "width" of the crowd can be up to about 16' wide and everyone will fit in the frame. But enter shorter distances... for example if they are only 10' away now the width of the frame drops down to 11' wide.
Hence if you can estimate how "wide" your group of people will be, you can estimate which focal length can get that crowd of people AND you can estimate about how far away you'll need to stand to get them to all fit inside the frame.
For group shots you probably want everyone in focus... front to back. So assuming you have many "rows" of people (and you may need a step ladder so you can see everyone's face in the shot if this is a big group) you'll want to set an f-stop that yeidls enough "depth of field" to get everyone in focus. This means you'll avoid the low and middle focal ratios and go for the somewhat higher focal ratios. E.g. use things like f/8 or f/11... but I'd probably avoid using focal ratios below f/8.
I also avoid using very wide lenses. If you use, for example, the EF-S 10-22mm lens and you're down at the 10mm end of it... you are EXTREMLEY wide. "Wide" lenses not only make the scene "wider"... they also make the scene "deeper". They "stretch" distances (e.g. the reason some rear-view mirrors say things like "Objects in mirror are closer than they appear" is because of this depth-stretching effect.) Commercial photographers will often use a "wide" lens for interiors because it makes the interior space seem bigger... roomier. But this has a bad effect on crowds of people because it makes the people in the front-row look a bit farther from the camera... but people in the back row seem a LOT farther from the camera. You don't want a group shot where the person in the back row who is really 20' away... seems like they are more like 100' away (you get a really really tiny person and can't see any detail in them.)
Good luck!
Tim Campbell
5D III, 5D IV, 60Da
