Actually, "I'm the best" is quite common among native speakers.
If "I'm best" is common among non-native speakers, then so be it. It's probably because non-native speakers can't delineate the difference in usage of comparatives and superlatives as well as native speakers.
"I'm good" is very common. So, it follows that "I'm best" is grammatically correct. But the superlative is usually singular, therefore it needs a determiner to indicate that. Many people can be good. But only one person can be the best.
If you look at this chart, it correctly puts "the" in front of the superlative.
Student or Learner