Well, the real answer is: whoever the writers want to make stronger for a particular episode for whatever reason. Power levels in the pokemon anime are inconsistent. Look at Pikachu as the prime example.
On the other hand, my totally biased answer is that Charizard is stronger. But probably not by much.
Charizard has better feats, namely: defeated the legendary Articuno.
Blazeape just defeated Electivire.
Also, we did see Charizard totally outclass Iris' Dragonite back in BW. There's 3 things that happened during the battle that shows it:
1-Charizard took Dragonite's Dragon Rush, his most powerful move, a move that had one-shotted other pokemon before (like Dawn's Mamoswine) head on, receiving it fully. Yet Charizard immediately got up, roaring, and with no visible signs of damage at all. Surprising and intimidating both Dragonite and Iris.
2-Charizard was much faster than Dragonite. When Dragonite was flying away from Dragon Tail, Charizard caught up to him much faster than the time it took Dragonite to get that far.
3-A single Dragon Tail from Charizard did significant damage to Dragonite. And that's a Dragonite that's known for taking x4 SE Ice type attacks for breakfast.
So yeah, Charizard it is.