[Stratos]

Adding levels of Fighter to your Ranger will only really give you some extra feats as well as Weapon Specialization. Rogue levels will give you rogue skills, sneak attacks as well as a number of other benefits. Both the Ranger and the Rogue uses stealth and both benefit from a high dexterity, so they mix very well.

The Cot PrC is basically a Fighter/Paladin hybrid and will give you Paladin-like abilities as well as bonus feats every two levels, just like a Fighter, with the excepton of Weapon Spec. Several of his abilites are charisma based, though, so you might want to start with a decent score here. Unfortunately, charisma is not the most important skill for either the Ranger or the Rogue.

Pick the Weapon Master PrC if you want to specialize in one kind of weapons and get massive combat bonuses while using those. Level 7 is a good stopping point if you pick the WM prC.