They are all great coins but If I were to go for one, I would go for polkadot, it's one hell of a great project and if am right in my prediction, polkadot will or might start a new trend in the crypto industry in this year 2021, BNB is also another coin I would love to invest in if i have some extra spare cash laying around as I think there's still a very large room for BNB to grow, but on a limited budget, I would put everything on polkadot.