Maniacal Laugh can easily turn the tide of the battle, and even potentially end the game in your favor, making it good for both a finishing trick and a stat-boosting card. In the case of the former, you should play it on the zombie you think has the biggest opportunity to finish the game, regardless of how powerful that zombie initially is. If you do choose to use it as a powerful but expensive stat-booster, however, you should play this on one of your stronger zombies to get the most out of it. Such zombies include those with great stats (either by default or boosted), traits, or an ability that activates by hurting a plant or your opponent.
However, while Maniacal Laugh is powerful, it is also very expensive, limiting your options on what you can play. Therefore, it may be better to use cheaper (although weaker) replacements instead, if you have them. If you want stat boosts, Vitamin Z and Loudmouth are cheaper options (especially on zombies with the Frenzy trait like Smashing Gargantuar and Vimpire). On the other hand, if you want to give a zombie the Frenzy trait, you have Lost Colosseum, Going Viral, and Coffee Zombie (the latter two giving all other zombies Frenzy too), although these are only possible as The Smash.
Against
If you see your opponent saving six or more brains for tricks, and there are no threatening plants on the field that may otherwise tempt them to play something like Locust Swarm or Hail-a-Copter, it is likely that Maniacal Laugh is about to be played.
The best counter would be to prevent or discourage your opponent from playing Maniacal Laugh. An early to mid-game rush or tempo deck is ideal, as your plants will constantly pressure your opponent so that they will have to fight your own rushes too, giving them no opportunities to safely play this trick. Brainana and Dark Matter Dragonfruit do a very good job later in the game, as the former sets your opponent's brain counter to 0 (for this turn), while the latter increases Maniacal Laugh's cost to 12, basically sealing any more uses of this trick (unless they go very late into the game). Black-Eyed Pea and Sportacus do penalize your opponent, although a mere +1/+1 or 2 damage won't stop your opponent from powering a zombie to massive levels in many situations.
If Maniacal Laugh is played, removing the boosting zombie should be your primary option, and fortunately, each class has a way of doing so (although they aren't equal in terms of efficiency): the Guardian and Solar classes have instant-kills, the Kabloom class has damaging tricks (although not very effective against a single powerful target), the Mega-Grow class can fight fire with fire by boosting their own plants and using bonus attacks, and the Smarty class has Bouncing cards to reset the zombie's stats (although it does not actually destroy the zombie). Unfortunately, you will have to take a hit from the boosted zombie first (unless the plant in front has very high health) due to the turn order.
If you don't think you will be removing the zombie anytime soon, you should play more defensively to survive. This is suited for the Guardian, Smarty, and Solar classes; the first has Soul Patch and shielding cards like Uncrackable and Template:PvZHLink, the second has Freezing cards like Template:PvZHLink and Template:PvZHLink to stall the zombie, and the third has healing cards. However, you must also have your own plants chipping away at your opponent too, as you can't defend and stall forever. And just like the former, you need to take a hit first, meaning that even this strategy can't save you if your health is extremely low.