I stayed at the Serena almost three years ago, so this info may be outdated. It was then the top-rated hotel in Kigali. Enjoyed my stay very much. Extremely friendly staff who can arrange car/driver/day trip/the works. Try to book a high floor, pool view. The Marriott is just down the street and looked close to completion then, but it just opened two months ago. Either place is probably a safe bet, but expect to pay at least $250-$300/night (unless you have Marriott points) and see lots of tour groups.