If I am a US citizen and I'm engaged and my fiance got deported from the US, what's the easiest way to try to legalize him?

There is only one way; it isn't easy; and it might be not open to your fiance. You need a good immigration attorney to determine if there is anything that can be done for your fiance - and to do it, if possible. As to doing it yourself... Let me ask you: if you needed a surgery, would you be asking on the internet how to do it - or would you be asking your friends, relatives, and co-workers for the name of a really good surgeon?

The best way I'd think is for you to file a Form I-130 petition for her after your marriage. The I-130 petition takes about 8 months to approve. Subsequently, she will undergo consular processing at a US embassy overseas. Now, given her deportation, you will need to apply for a Form I-212 Request to Apply for Permission to Enter the US in addition to a Form I-601 Waiver to cure the grounds of inadmissibility. Please note that it is essential to inquire into the specific grounds for her deportation and not all grounds of deportation can be waived. And even for those grounds that can be waived, there are different statutory requirements. You will find that hiring an experienced immigration attorney is the best way to go about this.

It all depends on why he was deported. We need more information to be able to assist you.

If he's in the US, marrying him is the quickest way to legalize him, granted he's otherwise eligible & the marriage is a good faith one. If he's outside of US then marry him too & file a waiver to bring him back depending on why he got deported. Certain facts in the case may not allow him a waiver or from entering US again.

Deportation means 10 year bar from coming back unless a waiver is granted by the USCIS. You should contact immigration lawyer for advise.