The one place in the Parc National du Haut Niger that you are guaranteed to see large mammals is the Centre de Conservation pour Chimpanzés. This is a French-run project in which chimpanzees rescued from the exotic animal pet trade are brought here and, over time, reintroduced to the wild. Don't get too excited about seeing wild chimps though, because visits are strictly controlled and the chimps that visitors get to see live in large enclosures.
Visits are limited to the chimps' noon and 3pm feeding sessions. The centre doesn't actively encourage visitors and it's a real mission getting out to their base (allow five hours during dry season in a high-quality 4WD from Faranah), so this begs the question as to whether or not it's worth the trip just to see some chimpanzees in what is, for all intents and purposes, a zoo. The answer to this question depends largely on your interest. If you have only a passing interest in seeing the chimpanzees, then it's probably not worth the effort. If, however, you are genuinely interested in seeing and learning more about chimpanzees in Guinea, then it's an easy decision. If you're in the latter category, see if you can arrange to stay the night at the centre (bring your own tent and food, including some foody treats for the centre staff, as they don't have the facilities to host visitors).
If you find yourself falling for the chimps, then you can always extend your stay. The project accepts a limited number of volunteers on six-month placements. You don't need to have a background in animal husbandry or conservation (though it would help if you did).
Any visit absolutely must be organised in advance both with the centre itself (and when they're out here in the forest they have no phone or internet so you might have to wait a couple of weeks for a reply) and with the park authorities at the Ministère de l'Agriculture in Faranah. Expect a very long day trip with at least ten hours in the car, on tracks so rough and overgrown that you will quite literally have to use a machete to hack your way through the forest in order to get there.