Many people say the British Empire was the most evil colonial empire, IMO though I'd say the Spanish Empire was worse.
They almost decimated the native Americans, and were amongst the first slave traders. They also had the Inquistion, which is something the British Empire never had on that scale.
Yes, the British Empire didn't treat India well and massacred Aboriginies in Australia, but it signed treaties with native Americans securing their protection, and didn't massacre them as the Spanish did.
Also, all of the other empires committed atrocities. What about the Belgian Congo, or experimenting on natives in Namibia? Or even starting the slave trade, which the British never did?