First: MEGA CHARIZARD WILL NOT DEFEAT MOST BASE FORM POKEMON. There's at least a good 100 Base Form Pokemon that can destroy Charizard.
Seccond, I take it you want a Death Battle Like Match. I watched Death Battle like matches, and the Pokemon ones/some others (too) often IGNORE/DON'T FULLY ACCOUNT FOR the stats (Think base Charizard having Metal Claw with 0 Atk Ev's is good ;-;/Basing Lucario's strength on it breaking a crane, not it's Atk stat/giving Pokemon ~10 moves) They AREN'T actually accurate, at all. (and anime matches just pull completely ridiculous ideas out of thin air, like Bulbasaur can learn Dig because it wanted to.)
So let me give something to go off of.
Serperior: Best chance of winning (what sumwun has shown us):
Serperior @ Choice Specs
Timid Nature
Ability: Contrary
EVs: 252 SpA / 104 SpD / 152 Spe
- Frenzy Plant
With this Serperior has a 79% chance of winning.
unless
Charizard has these held items: Choice Scarf/Focus Sash/(maybe other):
Charizard@Choice Scarf/Focus Sash
Modest Nature
Ability: Blaze/Solar Power
- Blast Burn
Charizard would have a 90% chance of winning with the item.
Ultimately, it depends on move-set and items, as well as training. Both have Ample Chance of winning, so to answer Would base form Charizard always win against Serperior in a 1v1?,
No, sometimes Serperior will win, sometimes Charizard will win, so Charizard won't always win.
Hope I helped!
:P