You should favour "on a par with", because it's the more commonly used expression, and has been around as such for a long time (longer than the game of golf, on which more in a moment).
It can be abbreviated to "on par with" for headline-style abbreviations. You should only do that in headlines.
In golf, it is normal to talk both of a player being "on par" meaning that they took the number of strokes that is the standard measure for the course in question, and also that they are "on par with" another player, meaning that they are currently (or finished) with an equal number of strokes. Because of this you will sometimes find "on par with" used figuratively for other contexts even outside of headline style and golf.
But since that remains a less common use, I'd still advise you to favour "on a par with" in your own writing.
Ngrams don't mean everything, but this chart suggests a strong preference for "on a par with" for both British and American use.
The Corpus of Contemporary American English finds 370 uses of "on a par with" to 348 of "on par with", or 322 to 272 if we exclude spoken news, sports magazines, and sports newspapers to reduce those using the term as golf jargon. Less strong a preference as found in ngrams, but still suggesting "on a par with" is the more common form.