A slightly better definition of "best" would be useful. Most results ? Best cost-efficiency ? Best results-losses ratio ? Best in comparision to its immediate neighbors (ie. the armies it fought against), or overall (in which case present-day militaries win hands down) ? Best straight-off combat prowess (often entirely different from the above) ? Best toys for its period ? Is it "better" to win by old-fashioned methods (in the "more challenging" sense), or to come up with newer and better methods contemporaries at least initially cannot counter ? Etc.

Ill-worded questions will probably only produce "mine's bigger than yours" -level brag-fests.