Your ranking at the end of the game is determined by how much total EXP you accumulated.  Three different items are added to the shop lists in Replay Mode depending on your final ranking in the previous game.  The best ranking cannot be achieved in your first game, because the necessary EXP just isn't available.  Ironically, though, some of the very best items to be added to the shops can be obtained only with the lowest amount of EXP.  You actually have to try very hard to get little enough EXP to obtain the lowest ranking, and that includes running away from lots of battles.  However, if you complete the game with the same ranking twice in a row, you will be awarded the next ranking up, which means that an S rank twice in a row will give you an L rank.  There is nothing higher than S, so it will cycle back to the lowest rank again.  This is a possibility for those of you who want the Monk Robe and can't seem to end the game with little enough experience.

Experience Acquired	Rank	Additional Items
14,500 and under	L	Recovery2, Medicine Wheel, Monk Robe
14,501 - 16,000	K	Flare, M9 Bayonet, M4A1 Clip
16,001 - 44,000	J	Protein Capsule, 9mm Hydra, M203
44,001 - 51,000	I	MP Boost1, Belt Pouch, Hammer
51,001 - 57,000	H	Lipstick, Pyke, Tactical Armor
57,001 - 62,000	G	Holy Water, MD Player, Javelin
62,001 - 66,000	F	9mm Spartan, Ofuda, Aya Special
66,001 - 69,000	E	R. Slug Shells, Firefly Shells, AS12 Shotgun
69,001 - 72,000	D	.44 Mongoose, .44 Magnum Rounds, Skull Crystal
72,001 - 75,000	C	Cola, .44 Maeda Rounds, M249
75,001 - 200,000	B	Recovery3, MM1, Airburst Grenades
200,001 - 400,000	A	MP Boost2, Hunter Goggles, Hyper Velocity
400,001 and up	S	Ringer's Solution, Eau de Toilette, Gunblade