Bolo stories are great, similar in some ways to the Asimov Robot series in some ways, except with Hellbore cannons, infinite repeaters, and nukes.
The Starship Troopers and Mote in God's Eye recommendations are great, I add some more:
Ender's Game - Bestest
John Ringo's Posleen series - Go Bun-Bun!
Harry Turtledove's World War series - Alternate history, aliens attack during WWII. Fantastico, stars real life badass Otto Skorzeny.
Edit: Ha! Shifty beat me to it on the World War series, that's the one he's talking about.