What books can you use?
Off-hand, the Templar strikes me as possibly your best bet. Assuming that Forgotten Realms campaign book is available, you need to take either Cosmopolitan or Education so that you can get the required ranks of Knowledge: Religion by level 5. Templar is a powerful class for Fighters; full BAB, nice abilities, and divine spells.
Divine Champion from FRCS is another nice PrC, in that you gain a lot and lose little. With both Templar and Divine Champion, you want to choose your deity carefully, as they both spend feats on the deity's weapon of choice. So, for example, if you prefer the greatsword, you might want Torm as your deity.
The Eldritch Warrior from Book of Eldritch Might II is pretty cool.
At level 13, a cleric can make a powerful melee combatant. Something to think about. If you don't want to be a full-fledged cleric, then even just one level of cleric is still quite nice. Two domain powers, a boost to your Will save, and the ability to use cheap Cure Light Wounds wands.
If the Psionics Handbook is available, nothing absorbs damage like a Psychic Warrior with the errata's Vigor and Improved Vigor. True, they don't dish out as much damage as Fighters, but they're competent warriors, can take more punishment than anyone, and have some nice psychic powers and feats. Even better, if you take one level of Egoist and the power Lesser Body Adjustment, then take the feat Trigger Power with it, you will be able to heal yourself at a very low cost (almost free). very valuable, especially if the group has no cleric.