I'll add my votes to the Honor Harrington series, as well as the Empire of Man, and also Moon's Serrano Legacy, all good reads.
You may also enjoy the
Assisti (sp?) Shards series, the first one is
1633 and is by Weber and Eric Flint.
David Drake's Lieutenant Leary series is pretty good, it starts with
Lt. Leary Commanding.
I also enjoy Jon Ringos Council Wars series, it starts with
There Will Be Dragons.