Ancalagon vs Gothmog

[The Might]

Ok firstly - and this is a different topic, but I'll still mention it - it's not clear that Ancalagon was the strongest of the dragons. A good case can be made that he was stronger than Glaurung - the only other competitor for that title - but then again Ancalagon is said to have been the strongest of the winged dragons. Whether this means that there were non-winged dragons who were stronger or whether Tolkien just happened to write this passage without thinking about such things is unknown.
What does indeed favour Ancalagon is his capability to fly and his hotter fire than Glaurungs, then again Glaurung had some tricks up his sleeve, being able to enchant people and bearing the title of father of all dragons.
So although I also rather think Ancalagon was stronger, it is never specifically stated. (or at least I am not aware of any such place)

Now to your question... it is much more simple. Gothmog is said to have been alongside Sauron Morgoth's most powerful and most important servant. Now again once could debate whether Sauron or Gothmog was stronger, but to your question this is irrelevant.

Comparing Ancalagon and Gothmog the winner is clearly Gothmog.