Tiger Airways (TR) usually wins by fare prices between KUL and SIN, they are always cheaper than Airasia (non AA sale period). However you'll be better off buying Airasia only when they have sale.
Normal periods, sometimes, MH/SQ can be cheaper than any of the budget airlines subject to the seasons in destination city. You know you can never get cheap tickets (by cheap: below USD50 all included) when its school holidays/national days of either country.
Jetstar is very much my choices usually but they have limited flights a day. And 3K can be cheaper than TR and AK sometimes.
On the other hand, Fireflyz, despite flying out of Subang (SZB) which is just 5km to downtown KL, is pricey as it is NOT a budget airline. But its turboprop plane is an experience in itself to try and it's spacious, clean and comfortable and comes with complimentary refreshments. With only 60 seat capacity, you can be assured of fast boarding/disembarking in more proper and orderly manner.
Local travellers can b*tches when it comes to boarding, despite having seat numbers, they'll still rush to the boarding gate.
Best advice: always compare between 3K, TR, AK, MH, SQ on the same date.
There's no one airline that is the cheapest all the time.. If you somehow manage to get USD50 fare all-in on MH/SQ, you better not think twice and don't hesitate to buy on the spot - it does happen but not THAT very often..
I'm in and out of KUL/SIN every other week so often that I can close my eyes and walk backwards and still make it to the gate faster than anyone else could. And I have flown on all 6 airlines here to SIN vv, but I never paid more than USD50 for a ticket, but sometimes I splurge giving the excuse peak period and no flights, then I get to fly on SQ at roughly USD150....
LCCT (low cost carrier terminal) and KUL main terminal is geographically next to each other separated by taxi-ing runway but to both terminals are only connected by road, ie: taxi or bus.
There is a KLIA shuttle bus for RM2 between main terminal and LCCT but departure time is really idiot. Like 15mins to 30 mins.
Connecting between airlines at these two terminals, you better allow minimum 1 hour to get from one to another.
And Subang terminal (SZB) is only 60km apart and takes just 45 minutes by car and not 70 miles. However there is NO BUS from SZB to KLIA/LCCT.
HOWEVER.. GOOD NEWS IS HERE!!
KLIA2 (designated low cost carrier terminal) which is beside the KLIA main terminal and can be connected by just walking, will be ready by end of 2012. The govt is still debating with the airport authorities whether or not to have aerobridges which could add hundreds of millions to the costs. The existing godown terminal will become a cargo terminal.
KLIA will undergo extensive renovations to a few gates and a third runway to accommodate Malaysia Airlines's A380 when it arrives in like.... many more years down the road