i ride mostly at the bike trial at the Bukit Timah nature reserve, the main trail is a loop about 6km long (fairly short for most who are used to trails in other parts of the world), but it's technical XC, i would think. The trail also extends beyond the main loop to other parts known as the more technical Butterfly trail and also the Woodcutter's trail, so you can find something that suits your riding preference, I'm sure.
besides that, there are trails at Kent Ridge park - the Redbull night DH race was held there not long ago
in the east of singapore there's the trail at Tampines which has a bmx track in there as well, but i've only been there once so i cant comment too much on that
i've been to a short DH run on Sentosa island before as well... not sure if it's still there though