You can say "excellent" by saying Yōuxiù (优秀) in standard mandarin (Chinese) and it sounds like 'yo sio' in English.
You can also just use the short form Yōu (优) to express excellent. Also, niú (牛) literally means "cow" but you can use it to mean "cool, awesome, excellent".
Another colloquial or informal way you can say zhēn (詹).
Other ways to say "excellent" are:
hěn hǎo (很好) which means "very good"
hǎojíle (好极了) which expresses something is extremely good
Yōuliáng (优良), meaning "excellent, exceptional"
Mào (茂) for expressing great quality
Nǐ zhēn niú - “You’re outstanding!”
Nǐ hěn kù (你很酷) - "You're cool"
Jí yōuliáng (即优良)- "That's excellent"
Here's a Youtube video of more Mandarin phrases that you might find useful in your pursuit of the popular Chinese language:
If you would also like to say "great" in Chinese then check this definition from the reference boards: