He did a fine job with his own private army during Sulla's civil wars, he managed Sertorius pretty well in Spain (consider at one point he was seriously undermanned in that war), he handily beat Caesar at Dyrrachium, he handled his commission to rid the seas of piracy expertly, he subdued rebellious tribes in Transalpine Gaul, and while Lucullus had done the lion's share of the work against MIthradates and Tigranes, Mithradates was still a considerable thorn in the side for Rome until Pompey finally subdued him. Certainly in the past he had managed to resurrect his fortunes at times when it seems he was finally finished-Pompey did what nobody else seemed to be able to, which was finish the job.
Well, as I said, he never won without overwhelming superiority. In fact the only time he truly faced adversity was in the early days against Sertorius (up until he had Sulla's faithful deputy Caecilius Metellus Pius's help, in fact) due to him and his men being out of their geographic and cultural element against a man with the support of pretty much the entire peninsula of Iberia, and against Caesar because Caesar is a daunting foe even when outnumbered.
I wouldn't say he really won against Caesar at Dyrrachium, it was more of a stalemate. I mean, he allowed a force a third the size of his own to pen him in, and didn't have the decisiveness to obliterate it. If he'd been younger, healthier, and sounder of mind, though...three against one odds would've made it fair to him. Pompey was good at mopping up an almost finished job, but he wasn't that great at grinding the enemy down or hitting them where it hurt unless he had expert counsel such as that of Metellus Pius or Labienus.
Cassius. He is remembered for Philippi, but he saved the Roman army at Carhae.
I don't know if I'd call him great, but he was certainly at least a competent general and administrator, effectively handling Rome's newest at the time Syria province as a quaestor in his early 30's.