Consider, before we get to AV, we have to hit, so what are our chances?
Melta = 2/3
Typhoon = 1-1/3 (2-2/3 for a pair of them)
Cyclone = 1-1/3 (2-2/3 for a pair of them)
Melta being AP1 also gets the nifty +1 to damage, so we can figure the chance to hit and kill assuming we get a penetration. Unfortunately glances cannot be figured here but they only give a minor bonus to the melta, but it does increase as the AV goes up.
Melta = 1/3 chance of kill on penetration.
LC = 2/9 chance of kill on penetration
TLLC = 8/27 chance of kill on penetration
TH Cyclone = 4/9 chance of kill on penetration (8/9 for two of them)
Typhoon = same as Cyclone.
This shows that the Cyclone and Typhoon actually have a better chance of killing if they get penetration than a melta does. Of course, this is an artifact of getting two shots.
Penetration chances (NOTE: at over 1/2 range melta penetrates like a Typhoon, but gets +1 on damage roll):
AV10: Melta 1/36 glance, 35/36 pen
LC/TH Cyclone 1/6 glance, 5/6 pen
Typhoon 1/6 glance, 2/3 pen
AV11: Melta 1/18 glance, 11/12 pen
LC/TH Cyclone 1/6 glance, 2/3 pen
Typhoon 1/6 glance, 1/2 pen
AV12: Melta 1/12 glance, 5/6 pen
LC/TH Cyclone 1/6 glance, 1/2 pen
Typhoon 1/6 glance, 1/3 pen
AV13: Melta 1/9 glance, 13/18 pen
LC/TH Cyclone 1/6 glance, 1/3 pen
Typhoon 1/6 glance, 1/6 pen
AV14: Melta 5/36 glance, 21/36 pen
LC/TH Cyclone 1/6 glance, 1/6 pen
Typhoon 1/6 glance, NO pen
Now instead of comparing a melta vs a lascannon (as is often done), let's compare actual weapon layouts on different units and see what the chances end up. For example, a MM Speeder vs AV14 has a nice 7/36 (19.4%) chance of getting a kill. A pair of them would double that to 7/18 (38.9%). Three of them would make it 7/12 (58.3%). The chance to kill on glance would add a couple of percentage points.
A TriLas vs AV14, it gets 10/81(12% chance), the TH Cyclone Term squad has a 4/27 (14.8% chance). If we assume that the Speeder has to spend at least one turn maneuvering to get that 12" melta shot, then it is actually more likely that the TriLas and TH Cyclone squad will get the kill.
The melta gets 32.4% vs AV10, the TriLas has a 61.7% chance, the Terminators would have a74.1% chance. A whole lot more likely.
This could take more grinding, but the main reason to take melta is that dice roll, winner take all to kill that Land Raider outright in one shot.