Though it lacks the Roman numeral designation, Metal Gear Solid: Peace Walker fulfills creator Hideo Kojima's promise that it would be the "true sequel" of the franchise. The story writes an important chapter for the series, describing the descent of Big Boss from hero to villain and the formation of Outer Haven. There's more than the full campaign too. 128 'Extra Ops' challenge missions add a pick-up-and-play mode perfect for the handheld format.
Peace Walker plays up to its billing as a true sequel as well. A number of improvements make the game more fun to play than prior PSP installments. And while the design presumes missions will be played cooperatively, it loses nothing in solo play. There's no reason to hopefully wait on the unlikely chance of it ever coming to PS3. Peacewalker shines as both a standout entry in the Metal Gear saga and a PSP game.
Runner-up: Valkyria Chronicles 2
Another sequel making the jump from PS3 to PSP, Valkyria Chronicles 2, like Peacewalker, brings everything that made it a great to the small screen. Its unique blend of turn-based orders mixed with real-time movement makes a great system for a handheld strategy game. And the watercolor on parchment illustration style looks as distinctive and beautiful as it did in the original.