I have a 12 gauge shotgun and i wanna hunt grouse,pheasant, rabbit whats the best ammo to use???

Since you are just starting and your shooting skills are not up to par, I would suggest #4 shot for rabbit and pheasant and #6 for grouse. Since you are an experienced shooter, you are more likely to have slower gun mount times and more misses that will lead to follow up shots at much longer range. The larger shot will give you more effective killing power at longer ranges. But will reduce you accuracy due to smaller and less dense patterns.... but you are always better off missing a target than you are putting a bunch of lead into your target that barely breaks the skin leaving a crippled bird that will eventually die of lead poisoning. The bigest problem with using shot size that is too large is damaging the meat at close ranges possibly yielding inedible game. Since you are an inexperienced hunter, that will be less of a concern. As you gain confidence in your shooting you can move up to a smaller shot size to provide for more consistent kills at a wider variety of rangers.

I use Federal Premium #4 or #6 and a modified cylinder all 3 game animals on your list

I use 32gram loads of no6. No problems.

i use small game loads

I love pheasant hunting and have done so for years and have always used #4 bird shot, which also works on bunnies. I have only ever hunted mountain grouse with a .22lr so I couldn't say what to use in a shotgun for whatever type of grouse your going after.

Use 5 or 6 for Grouse & Pheasant and 6 & 7 1/2 for Rabbit these should produce great results.