My favourite combo (from Magic: the Gathering) is definitely Sporesower Thallid, Doubling Season, Spore Flower, and Aspect of Mongoose (Fungal Bloom can replace either of the first two if you don't use Aspect of Mongoose) for a constant fog machine (enemy monsters can never hurt you).

Well mine varies depending on what deck I'm using, their all for yuigoh though.

Plant Deck; 1, Lonefire blossom, effect- special summon Gigaplant from deck, next turn, re-summon gigaplant for effect, effect- special summon lonefire blossom from graveyard, lonefire effect, special Tytannial, Princess of Camellias. And continue in this fashion, its also nice to have a Pollinosis set, Wall of Thorns, and Black Garden in play after summoning Tytannial.
Also another good combo is Twilight Rose Knight, - effect special lv 4 plant monster from hand and synchro summon Black Rose Dragon. ^_^
Black Rose Dragon is currently limited to 1, however my record so far is bringing it out 3 times in one duel.

Burn Deck; 1, Level Limit B or Gravity Bind in play, Raging Flame Sprite in attack position, Special Lava Golem to opponents side of field by tributing 2 of opponents monsters. Activate Mask of Restrict.

Fortune Lady Deck; 1st, set Interdimentional Matter Transporter or Compulsory Evacuation Device, and a random monster set. Next turn, Activate Secret village of the Spellcasters, Toss Fortune Lady Watery for One for One and Special Fortune Lady Lighty from deck. Flip Interdimentional or Evac, Lighty's effect activates special Fortune Lady Darky from deck. Darky attack opponents monster, Darky's effect activates special Watery from gravyard, Watery effect activats, draw two cards. Normal Frequency Magician, Synchro Frequency magician with Watery into Brionic, Dragon of the Ice Barrier.

Theres a few others that I like but I can't be bothered typing them up.

Also heres one my partner uses in a duel with the HERO class monsters;
Deck is fast synchro summon and draw power focused purely on Stardust Dragon and Majestic Star Dragon. Majestic Star Dragon can be brought out in one turn with this deck, not needing anything in the grave or on the field! Anyways, wait until the newly popular Elemental Hero Absolute Zero is summoned, take whatever damage I have to, then, bring out Majestic Star Dragon, attack Zero, when Zero is removed from the field, it's effect activates and destroys all of your opponents monsters. You can activate Majestic Star's effect to negate the effect of Zero and nuke your opponents' entire field! It's an official ruling due to the fact that it doesn't state on the card where your opponents' monster has to be, it's been checked.