Important notice.Almost all hotels in Nigeria require you to pay before you get your key. This applies even to the Sheraton and the Hilton. Typically you are requested to pay 125% of the room rate and you will be refunded when settling the bill at your departure. If you stay more than one night you need to keep the credit up. However, paying this deposit by credit card can leave you open to subsequent fraudulent use of your details.
The Transcorp Hilton in Abuja is 5 star and a top ranked hotel in Nigeria. It's comparable to nice hotels in other developing countries, although well below the standard of European or North American four- or five-star hotels. However, if you decide to visit the hotel bar, be warned that the single women who seem so interested in you are almost certainly "working." This is true of many hotels that cater to international clients.
In Port Harcourt, the Meridian is quite decent. It's a tad bit expensive but your money's worth is guaranteed.
In Lagos, the Sheraton Hotel and the Kuramo Lodge on Victoria Island are ranked 4 star. You can also try the Eko Hotel & Suites adjacent to Kuramo Lodge. It's definitely a favourite for tourists and foreigners.
In Kano, you can have a aircond room in Tahir Guest Palace, Prince hotel, or one of many small hotels. The Green Palace Hotel in Kano is awesome. It is roomy, not as isolated as the Prince, and just has a pleasant ambience.