Really, your Kirlia should be so powerful in Sp Atk that defense shouldn't mean anything, but if you want one that's good in both Sp atk and defenses, you're stuck, because Psychics as a general rule either have one or the other, or are uber.
For beating the game though, you can't get much better than Gardevoir (Kirlia's evo) because of the movepool and stats it has. Just evolve your Kirlia and watch it rip through everything in the game. Alakazam is another choice, with better Sp Atk, but it doesn't have as good a movepool as Garde and its defenses are bad enough to make Gardevoir's look good. Just give Garde Psychic and Thunderbolt and whatever it can't beat with those moves you just beat with another pokemon.
Grumpig is a more defensive option though, but it hasn't got the movepool of Kirlia and learns Psychic a whole 13 levels later than her, as well as having no other good specials to learn before the elemental punches found in the Frontier.
