If you're savvy enough to have gotten the Liberty Pass and caught Victini right away rather than waiting until later, you'll find no better Pokemon to use on your team for most all of the game. There's a reason that Victini is considered a legendary Pokemon, and its fantastic stats and great arsenal of moves is the result of that status.
It starts off at level 15 with Confusion, Incinerate, Quick Attack, and Endure (sadly you need to wait until later to reteach Searing Shot, which is an amazing attack). Confusion is Victini's main attack, since Incinerate is notoriously weak. At level 17, it learns Headbutt, which is nice and strong at this point in the game. It learns Flame Charge at level 25, which raises Victini's Speed afterwards and should replace Incinerate. At level 33, it learns Reversal, but that's hard to pull off with Endure and vulnerable to random Quick Attacks. It picks up a fairly strong Flame Burst at level 41, Zen Headbutt at level 49, and then the Zap Cannon of Burn at level 57: Inferno. It learns moves all the way up to level 97, where it learns Overheat (hardly worth the hassle). Flare Blitz is alright at level 73, but that's still such a long way away you can't even look at it.
One problem you may notice in looking at those moves are that it only learns its better moves way later in the game. That's why you need to be savvy with investing some TM moves into Victini's movepool! Some of the helpful TMs you have access to right now are Retaliate, Grass Knot, Thief, and Rock Smash. Switch them around to your liking, but definitely keep its default Confusion. Later on in the game, some good offense moves are Fire Blast (definitely, although lack of PP hurts), Thunderbolt, Shadow Ball, Brick Break, Charge Beam, and Focus Blast, while some good defensive or support moves to consider are Sunny Day (works great if you throw in SolarBeam!), Light Screen, Thunder Wave, and Substitute.
Victini has access to its better moves like Psychic, Flamethrower, and Psyshock after you've beaten the game.
Victini's stats and diverse movepool make it an incredible Pokemon throughout the game. Though it does level up rather slowly, it can fall behind in levels and still be more effective than probably any of your Pokemon, particularly early on in the game. It's arguably too good to use in-game, and some people have a stigma about using legendary Pokemon while playing through the game, so consider that before considering whether or not you want to let Victini pave your way to victory.