Ah, Mild Nature, that works very well for you - your Sp.Attack increases at the cost of some of your Defense. The Sp.Attack raise is very useful, and the Defense drop isn't really too much of a loss since Raichu's defense is typically too low to stand up to much abuse anyway.
With that, I'd go for a moveset like this:
Raichu @ Life Orb/Focus Sash
252 SP.ATK/252 SPEED/6 HP
Mild
Static
- Thunderbolt
- Grass Knot
- Focus Punch
- Substitute
Special attacks are the main focus for this set since your Nature supports that the most. Thunderbolt is your STAB move of choice here - you CAN use Volt Tackle, although the most prominent threats that fear Thunderbolt (Skarmory and Gyarados mostly) don't tend to fear the physical Volt Tackle as much due to massive Defense for Skarmory and Intimidate on Gyarados, plus with the Nature boosting your Sp.Attack odds are Thunderbolt would outdamage Volt Tackle anyway if your Sp.Attack is significantly greater than your Attack. Grass Knot can bring the pain to Ground-types easily - since most of them are very heavy, Grass Knot will hit them extremely hard; Rhyperior and Swampert especially won't be able to last long against you. Finally, since you mentioned Focus Punch and it makes for a great option, it's there along with Substitute - Sub up when you predict something is about to hit you with a status move, then smash them with a crushing Focus Punch for massive damage. Blissey and Umbreon especially fear this combination.