Magmortar theme deck (Secret Wonders) has, well, Magmortar, so it's probally the best. But, just to let you know... all theme decks are bad. Just get cards from theme decks, then make decks with them.
Tier 1 decks: Gardevoir/Gallade (G&G) - more Gallade oriented, the strategy is simple, just do tons of damage with Gallade, then bring out Gardevoir Lv.X and Bring everything Down.
Magmorar/Typhlosion/Delcatty (Fire TRUK) - The original version of Magmortar decks, it's now overshadowed by Magken, but the strategy is to discard Energy with Delcatty, Firestarter it back to Magmortar, so your really attaching 2 Fire Energy per turn.
Magmortar/Blaziken/Claydol (Magken): The new version of Magmortar, Magken has Blaziken Power Keepers instead of Typhlosion, Claydol instead of Delcatty, and Blaziken Great Encounters as a secondery attacker.
Gardevoir (PLOX) - still has Gallade, like G&G, but here it's waaaay more Gardevoir oriented, and it has Claydol. The strategy is to Psychic Lock opponent's, use Team Galactic's Wager to reduce their hand size, then stop their Claydol (and all other Powers too).
Banette (T2 Banette) - strategy is to get Banette out Turn 2 with a Banette in the discard (pretty easy) and hit for 80. 160 against anything Psychic weak with Lake Boundary in play.
Ho-Oh/Togekiss (Skittles) - put Energy onto Ho-Oh with Togekiss and pwn... lol.