Danum ValleySabah of Malaysia.
Minivans between DVFC and Lahad Datu office go three days a week (Jan 2017: Monday, Wednesday, Friday, 3:30PM from LD, 8:30AM from DVFC). Tour operators in KK and possibly in other Sabah towns may offer touristic packages, which are the same two options (plus commission and possibly transportation to Lahad Datu), as the only these two organizations (DVFC and BRL) run two different facilities in Danum Valley. Lodging in both places are heavily overpriced but other fees in DVFC are more or less reasonable (Jan 2017: dorm RM96, sheltered plank bed RM78 , transportation RM120 one way/person, ranger RM20/hr, compact camera/DSLR RM5/RM100, nightdrive/sunset-sunrise drive RM140). DVFC have a system of poorly labeled trails (and few schematic maps at the center show it badly) for 1-7 hr of walking time, access to most of them are permitted only with ranger. Observation tower are closed for maintenance for a indefinite time (Jan 2017), few smaller towers on the trails for animal observation are closed to tourists. There is no canopy walk in DVFC, in contrast to BRL (for must more cheaper but satisfying option of canopy walks see Sepilok's Rainforest Discovery Center). There is small chances to see big mammals (in contrast to many species of invertebrates) on the tracks, but in the clearings around DVFC few of them may be easily spotted (eg. sambar and muntjak deers, bearded pigs, long-tailed macaque, civets, also as monitor lizard and birds).
The right place to visit untouched Bornean rainforest at it best - lush, wet, and dark. Unauthorized access to Danum Valley Conservation Area (DVCA) is forbidden and two checkpoints on the dirt road from Lahad Datu (town, from which trip to Danum Valey starts) enforce the policy effectively.
Flora and fauna
There are two options to visit Danum Valley: Danum Valley Field Centre (DVFC) is older and established mostly to researchers who want to study any aspects of Bornean rainforest; and Borneo Rainforest Lodge (BRL), gemmated from DVFC to decrease tourists' pressure from the latter. Offices of both organizations are located neighborly at Fajar centre block 3 of Lahad Datu town (GPS WGS84: N5.03007 E118.32604), and required to visit as the only way to reach Danum Valey is by authorized minivan arranged there. Reservation must be done in advance by phone/e-mail, if you will not bet on your luck to get a vacant place.