Welcome! Not that it's the most popular but my M1 Garand at 30-06 has been a vary lethal platform for a vary long time. More apt for two legged animals than four legged.
I think the Marlin Model 60 is the most popular rimfire rifle. They had sold over 11,000,000 years ago. It's cheap, accurate and has been made for many years. I bought mine new in 2002 for $108 at Walmart and I think they now sell them for $168 not on sale.
Anyway, the answer to your question really depends on what you mean by popular and over which time period? I assume you mean private firearms and not service firearms. Is it the most sold over the past year, decade or century? Do you mean a specific manufacturer and model (including specific caliber) and or type? Even once you nailed down a precise question, getting a precise answer will be difficult. In conclusion, all you get from this forum are personal opinions and not facts on such a question.