Of those, I'd do the top loop, but extend it to take in Nethy Bridge and Loch Garten - the woodland around there is beautiful, and you can easily navigate back onto the track shown. Just leave plenty of time to ride the valleys through the Cairngorms proper. I haven't ridden the two shown, but find in general you end up with a mix of boggy and and rocks/boulders. Find for walking - not easy to ride.
If you have a spare half day, there's also a track from just south of Aviemore (Burma Road?) that takes you over one of the hills and down to Carrbridge which is worth doing. The climbs a pig, but the route down to the other side and back to Aviemore (you can do some of it off-road) is wonderful. Not 'epic' but very picturesque.