Which coin is best for long term investment ADA or polkadot because these coin has behind good team.
Please don't obsess yourself with long term investments. Especially if you are talking about altcoin investments, not bitcoin investment.
With altcoins, you can hold them in the bull market, for a few months and take profit. The bull market for altcoins won't last long and their prices are not strong like bitcoin. In bear market, bitcoin is the main attraction for investors and their capital. Without money, altcoins will slowly lost their values.
ADA and Polkadot are good for you in next 6 or 7 months or a few early months in 2022. After that, you can exit all altcoins and focus on bitcoin or stable coins.