What is the best reflective tape? That's a hard question answer because there are actually many different types of reflective tapes that are each better for different applications. With that being said, many would consider the brightest reflective tape to be the "Best". The brightest reflective tape by far is Oralite SOLAS Tape.
SOLAS stands for "Safety Of Life At Sea" and is a Coast Guard approved film to be used in maritime environments. This is because a maritime environment is the most dangerous at night when you are searching for someone or something (such as a boat). This tape can be seen for thousands of feet (some unofficial reports point to up to a mile away in ideal conditions). This is because the candela score is approximately 1000 (candela is the official metric for measuring reflective films). A car license tag is about 75 for reference.
SOLAS is the brightest tape but may be overkill for traffic or vehicle applications where you don't necessarily need to see a person or vehicle for as far as SOLAS. Also, SOLAS only comes in one color which is a silver/grey color. In my professional opinion, I would consider Oralite V98 to be the best tape for vehicle application due to both its night time and its daytime visibility combined with its conformability.
Oralite V98 is an ultra-bright tape for nighttime reflectivity and also very vivid during the daytime. This film is conformable so it can actually go over seams, cracks, and treadplate eaisly. Another reason why it's so popular is that its adhesive is one that allows it to be repositioned. If you apply it and it turns out not to be perfectly straight or in the wrong place, you can take it off and reapply (this is obviously within reason, the material is repositionable as long as you don't press it in or let it sit for too long). This material also comes in many different colors to best fit your needs.