If you're flying into KL and only staying for a day or two, then the best value card is the KL TravelPass (www.kliaekspres.com). Included in the cost of a single (RM70) or return (RM115) transfer on the KLIA Ekspres train with the international airport is two days of rides on the city's LRT, MRT and monorail lines.
If you're staying for an extended period in KL, consider the prepaid MyRapid card (www.myrapid.com.my), valid on Rapid KL buses, the monorail and the Ampang and Kelana Jaya LRT lines. It costs RM20 (including RM5 in credit) and can be bought at monorail and LRT stations. Just tap at the ticket gates or when you get on the bus and the correct fare will be deducted.
Each time you reload this card with credit, RM0.50 will be deducted as a reload fee.
The Touch 'n Go card (www.touchngo.com.my) can be used on all public transport in the Klang Valley, at highway toll booths across Malaysia and at selected parking sites. The cards, which cost RM10 and can be reloaded with values from RM10 to RM500, can be purchased at most petrol stations, KL Sentral, and the central LRT stations KLCC, Masjid Jamek and Dang Wangi.