I've been going back through the Man-Kzin Wars books lately, and they still hold up. If you're already a fan of Larry Niven's Known Space universe, they're definitely worth reading.
The Kzinti make great enemies because they're not just "evil space cats." They're proud, aggressive, and absolutely convinced they're the superior species. More than once they underestimate humans, and it usually costs them. I also like that each book explores different parts of the war from different authors, so you get a lot of variety without losing the overall feel of the setting.
For me, the Kzinti are still one of the best alien races in science fiction. They have a believable culture, a strict code of honor, and just enough arrogance to get themselves into trouble. If you enjoy military sci-fi with good world-building, the Man-Kzin Wars series is an easy recommendation.
Killswitch
*Quantum Override*