Charizard is actually the worst combo with Heatran lv. X in my opinion because of four reasons:
1.) He is a stage 2 with a bulky retreat cost.
2.) His only attack requires 4 energy. Even with Heatran lv. X eventually retrieving them, it will still take you 4 turns to power up, which is way too slow in this or most any other format.
3.) He lacks the ability to OHKO most stage 2 Pokemon, which really hurts.
4.) He is double weak to water, meaning the most popular deck in the format (Kingdra) will OHKO it every single time with a single energy.
I don't like Moltres much either (I know you didn't mention it, but I'm bringing it up as a point of comparison), but it is actually a lot better than Charizard because it hits for 100 as a Basic Pokemon for only 3 energies (which is still slow, but faster than Charizard) and it can recover energy from a downed Pokemon when you bench it on a flip.
I think Magmortar lv. X is probably the best combination with Heatran lv. X at the moment because it abuses both aspects of the card (Burn and discard retrieval), with MD Infernape coming in second due to its ability to get OHKOs consistently. When Galactic's Conquest is released I think the Blaziken from that set comboed with the GE Blaziken (and Heatran lv. X of course) will make a potent deck as well (GC Blaziken has an auto-Burn Power, hits for 100 for 3, can Clutch for 2; GE Blaziken is simply a sick sniper, taking Claydols in one hit; both have 130 HP and a low retreat of 1).