The Lilycove City contest hall has a berry blender with 3 NPCs, making that one the best, as the more people you have at a blender, the more the quality increases. Unless you have 3 friends with some skill at the berry blender minigame, it probably best to just use the one with the 3 NPCs. NPCs don't have set berries they use every time, if you use a rare berry, the NPCs use rarer berries. The NPCs do have a selection of berries to use, and they are more likely to use their rare ones if you use a rare one. Pretty much, use the berry blender in Lilycove City with 3 NPCs, and use your rarest berries.
If you want the best kind of Pokeblocks, you'll want to use rarer Berries, which automatically makes the other blenders use better berries.
Source: Serebii, and my experience with all things Hoenn.
Note: If you're playing Emerald, just wait for the Blend Master, he always uses rare berries and always mixes perfectly. You need to beat the champion before he appears though.
Hope I helped.
