Strong emphasis:
* Absolutely
* Completely
* Totally
* Utterly
* Thoroughly
* Plainly
* Positively
* Outright
Negative connotation:
* Outrageous
* Appalling
* Shocking
* Dreadful
* Awful
* Terrible
Other:
* Frankly
* Honestly
* Truly
* Simply
* Purely
* Merely
* Indeed
* In fact
The best synonym will depend on the specific context in which "downright" is being used.