Best plugin to improve website speed

I am a website developer. I tried a few plugin (those plugins have high download & good review) to improve website speed but all plugin clash with my theme and made my website become error

Hummingbird makes my clients’ website speed improved to average result as per GTMetrix, I am more than satisfied with this result because I am using shared hosting for my client

No clash, no error, everything works fine to my clients’ website. Highly recommended