AnimeKing

Crocodile did in fact lose to Luffy, but it was after he defeated him twice and almost outright killed him.

Luffy found his natural weakness and was able to defeat him and yes while this was a Luffy with no Haki or Gears. Crocodile I would argue was still his most toughest opponent Pre Ts.

Also I wanna add that I did hear (don't know how true it is) Crocodile was supposed to be one of the tougher opponents for Luffy as Oda didn't see the story lasting this Long.

Nonetheless whether it's true or not. Oda has definitely realized his mistake with Croc and has been portraying him to be very powerful.

In Marine ford he Went on to briefly fight against w Doffy, Mihawk, and Sakazuki. Crocodile was all over Marine ford and he left with only minor injuries despite doing all he did.

Fast Forward we learn about Cross Guild.

A Group mainly consisting of three heavy hitters. Buggy, Crocodile, and Mihawk

Buggy became a Yonko as he is seen as the “Leader” but let’s look at Crocodile, He now has a bounty of $1,965,000,000 Berries.

• He has a substantially higher bounty than any 1st Commander we’ve seen so far barring Beckman.
• Buggy became a yonko essentially having only him and Mihawk as his “commanders.”

I think it's easy to see that Oda has been trying to rebuild Crocodile's stock as even he probably believes Luffy defeated him too early.

But even despite the defeat as I said Crocodile still was probably Luffys toughest pre ts Opponent.