The Federation is strongest diplomatically.
The Klingons are strongest militarily.
The Romulans are strongest clandestinely.
All factions have strategic geniuses among them. The Federation holds several wild cards in that it is inclusive and has many races working together for a common goal. The Klingons and Romulans are exclusive and tend to subjugate other races as opposed to treating them as equal members as the Federation does. Eventually, the Fed even makes a staunch ally of the Klingons. Well, at least that's the way it happened in the old timeline.
"One of the many, the proud, the friends of Zardoz".