1. Tywin lannister - everyone agrees he's a great field commander and we also see him win some bone crushing victories in the book- blackwater, green fork, reynes, tarbecks. But his struggle with robb stark and the fact that he was numerically and strategically stronger in all those battles make it difficult to easily name him the best. He did however earn my respect when he withdrew his army to harrenhal and plundered the riverlands with light cavalry to lure stark to him and earned my doubts when he blindly chased robb to the westerlands and nearly got trapped
2. Randyll tarly- everyone says he's one of the best generals but we haven't really seen lord baldy in any real action except ashford.
3. Robb stark- alexander the great of westeros. Continuously defeated superior forces in dire strategic situations( whispering woods and so on....), master of surprise and deception and an extraordinary military mind. He defeated tywin lannister and jaime lannister in battle at the age of 16!
4. Stannis baratheon- Probably the most underrated character of the series. He has the best track record of any men in this. Always the underdog and always the victor.( siege of storm's end, castle black ) . Plus he's also a great admiral, defeating the iron fleet( the best sailors in the world even if not the best ships), capturing dragonstone, old wyk.
Plus he has one thing on his history no one else on this list has- he has personally fought in battles.
Robb and tarly have fought in several battles, but no mention of tywin with sword but stannis outdoes them all in personal valor, the mirror image of Richard the 3rd