The only really answerable question in the format we have here is, "How do I compare similar lenses to decide which is best for me?
Let's look at the three lenses in question:
- The Canon EF 70-200mm f/2.8 L USM was introduced in 1995 when 35mm film was still the standard and most of us never dreamed we'd be shooting digital cameras for anything other than high end commercial work.¹ It was designed to work within the expectations of film shooters and the limits of that medium. (No one used to blow every frame of film up to 60X40 to look at it, yet that's what I do every time I pixel peep at 100% on my monitor!) It was a very good lens for its time. It's still a pretty good lens. But other, more recent offerings give higher optical performance not only in terms of "sharpness", but also in terms of vignetting, flare resistance, lens coatings, etc. They also offer Image Stabilization, which had not yet been introduced by Canon in early 1995 when this lens was rolled out.
- The Tamron SP 70-200mm f/2.8 Di VC USD was introduced in 2012. It's a lens designed for the higher expectations of digital photographers. It offers "VC" (Vibration Compensation), Tamron's version of Image Stabilization. But it is also a 'third party' lens that carries the potential for compatibility issues with future Canon bodies. It's a very good lens, but not quite up to the standard of the Canon EF 70-200mm f/2.8 L IS II which was arguably the best mass-produced zoom lens in the world when it was rolled out in 2010. The Canon "II" gives excellent performance all the way from 70mm to 200mm at every aperture setting, even wide open. The Tamron keeps up until the longer focal lengths, where it fades a bit at wider apertures compared to the Canon "II". This 2012 Tamron is equal or better optically than the 1995 Canon at most every measurable metric.
- The Tamron SP 70-200mm f/2.8 Di VC USD G2 was introduced in 2017. Although the optical formula is similar to the previous generation, it has tested noticeably better at most reviewers and labs. Many theorize that improved manufacturing methods and mechanical design have allowed Tamron to produce the newer lens to higher tolerances. It has even tested slightly better than the Canon "II" at some focal length and aperture combinations by some reviewers and labs. Additionally, it is compatible with the Tamron 'TAP-In' console that allows the end user to update firmware without sending the lens in to a service center. This helps ammeliate a lot of the concern with using a third party lens.
Some of the questions you need to ask yourself are:
- At what focal lengths do I plan on using a 70-200/2.8 the most? For most of us, we tend to shoot the most at the extremes - 70mm and 200mm - of the zoom range, but that may or may not be the case for you. If you are not concerned with 200mm near as much as you are with 70mm-135mm, then the weaker performance of the two older lenses at 200mm is not as much of a concern. If you plan on using it a lot at 200mm, particularly at f/2.8, then the newer G2 offers better optical performance than the older 2012 Tamron and 1995 Canon lenses.
- Do I plan on using a teleconverter/extender with the lens? If so, performance at the longer focal lengths is even more critical, as TCs magnify every flaw in the center part of the FoV of the lens in front of it. We don't put a TC on a 70-200mm lens to use it at 98-200mm (1.4X) or 140-200mm (2X). We already have those focal length ranges without giving up one or two stops of aperture and a bit of optical performance! We put a TC on a 70-200mm lens to use it at 200-280mm (1.4X) or 200mm-400mm (2X).
- Is the absolute best optical performance necessary for my intended usage? If all you plan to do is post 2-4-8 MB images on the internet, it probably is not. If you intend on doing a lot of significant cropping or displaying at large sizes, such as 30 x 24 inch prints or beyond, it probably is.
- Will I take most of my photos with this lens using autofocus, or will I mainly be using it for carefully manually focused shots? It doesn't matter how sharp a lens is, if your subject is out of focus. Camera manufacturers lenses tend to AF better than their third party counterparts, but there are exceptions. The Tamron G2 seems it might be one of them that is just as accurate and consistent with AF as Canon's recent 70-200 lenses. The TAP-in console also allows the G2 lens to be "dialed in" to a particular camera body at several focal lengths and subject distances to compensate for minor differences between bodies and lenses due to manufacturing tolerances. If the Canon camera body you are using has AFMA capability, it is still a bit more limited in scope than the adjustments offered by the TAP-in console with the G2 lens.
- Will I be shooting in situations that would be helped by Image Stabilization? Keep in mind that IS only helps with camera/lens movement when using longer shutter times. It does nothing to compensate for subject motion. Only shorter shutter times (faster shutter "speeds") can do that. The 1995 Canon has no IS. The 2012 Tamron has VC (Vibration Compensation) rated at four stops. The 2017 Tamron G2 has VC rated at five stops. That means that theoretically, with a static subject, you could shoot handheld with a shutter time twice as long with the G2 before blur caused by camera movement would be the same amount as with the 2012 Tamron. On the other hand, IS isn't very useful for shooting action or sports where your subjects are moving fairly fast. It all depends on what you are shooting how much IS matters to you.
- Do I plan on shooting subjects at very close distances? The 2017 Tamron G2 has an impressive 37.4" (950mm) Minimum Focus Distance, compared to 51.2" (1300mm) for the 2012 Tamron and 59" (1500mm) for the 1995 Canon. While neither of these lenses is anything approaching a Macro lens, shorter MFDs allow you to make small subjects larger by shooting from closer distances than longer MFDs.
- How important is price to my decision? Will getting a more expensive lens that offers things I don't really need limit my ability to buy other things I do need? Will it limit my ability to travel to locations to shoot what I want to shoot with this lens? Or do I truly need the absolute best optical performance money can buy?
Only you can answer each of these questions and weigh the various answers to come to a decision about what is best for you. Each of these lenses are the best choice for someone, or the market would not support them.
¹ Ironically, high end commercial work was one of the last segments to even begin to go digital. There is still much large format commercial work done with film, though not as much as even five years ago.