I think it's too early to say Arbitrum (ARB) is the most promising L2 project because ARB is a new altcoin and its resilience against the ups and downs of the crypto market is untested. there are still a number of L2 projects such as polygon (matic) which have proven promising and proven to be resistant to crypto market conditions, apart from that MATIC's position at cmc has also continued to rise.
It's definitely too early to judge ARB to be a rising star or anything as it's too new in the market and it hasn't been doing very great after the launch even though it was too much hyped when the airdrop was about to happen. On top of everything, it's an altcoin and altcoins are always too risky and anything can happen at any time with them.
So it's probably the best practice to just take it as any other altcoin and invest only what you can afford to lose. Investing in altcoins is way too risky and can pose serious threats to your capital, especially if the token is new.