pikachu is a pretty well-rounded character. everyone was making a big stink about him when the game first came out but i feel like all the hype around the character has really died down. i still think he's up there with the best in the game. probably my next best after mewtwo (and maybe shmewtwo). below i've listed what i believe to be some of his optimal combos.
*Note 1: -> is forward, <- backward, ^ up, V down. i separate the moves in a combo with commas. periods between button presses indicate that i consider them part of the same move (still entered in sequence, not simultaneously).
*Note 2: i define an optimal combo as the most damaging sequence of moves that are confirmed by a particular initiating move. i've listed the combos below by the initiating (confirming) move.
*Note 3: by inputting forward and y at the same time, you trigger a more powerful version of the move. i believe it may also allow you to extend your combo by another hit before triggering phase shift. will do more experimenting with this. please comment if you have anything to add in this regard.
I) combos
1) y confirm
a) y.y.y.x, ^ a (139 damage)
b) y.y.y.x (wall splat), (wait) x, Va (219 damage)
2) ->y confirm
a) ->y, x, Va (153 damage)
b) ->y (wall splat), (wait) x, Va (201 damage)
3) midair y confirm
a) midair y, ->y, x, Va (183 damage)
b) midair y, Vx (wall splat), ->y, x, ^ a (273 damage)
4) Vx confirm
Vx (wall splat), ->y, x, ^ a (246 damage)
5) x confirm
a) x, Va (143 damage)
b) x (wall splat), (wait) x, Va (229 damage)
6) midair x confirm
a) midair x, ->y, x, Va (183 damage)
b) midair x, Vx (wall splat), ->y, x, ^ a (262 damage)
7) ->a confirm
a) ->a, a (105 damage)
b) ->a (wall splat), x, Va (195 damage)
8) midair a confirm
a) midair a, midair y, x, Va (173 damage)
b) midair a, midair x, x, Va (169 damage)
c) midair a, midair y, Vx (wall splat), (wait) x, Va (224 damage)
*at times impossible to connect with midair y after midair a for combo 8a. depends on your distance from opponent after you land from the midair a. this is why i included combo 8b (midair x can be landed in these cases).
9) a+x confirm
a) a+x, ->y, x, Va (121 damage)
b) a+x, ->y (wall splat), <-a, Va (161 damage)
II) Synergy Burst Combos
*Note 4: only listing the combos unique to synergy burst mode in this section. otherwise the above combos remain optimal.
III) strings
*Note 5: pikachu's strings generally involve true comboing into ^ x and then following up with y+b or ->a. "/" denotes the point at which your opponent can break out of the string either with an attack or jump.
1) ^ x confirm
a)^ x / y+b (60 + 120 damage)
b) ^ x, ->a (wall splat), x, Va (60 + 195 damage)
2) midair x confirm
a) midair x, ^ x / y+b (124 + 120 damage)
b) midair x, ^ x / ->a (wall splat), x, Va (124 + 195 damage)
3) midair a confirm
a) midair a, midair x, ^ x, y+b (154 + 120 damage)
b) midair a, midair x, ^ x, ->a (wall splat), Va (154 + 155 damage)
*after your opponent has been conditioned to hitting attack or jump to break out of the string, try following up with a counter attack or quick normal.