Wow, I have quite a collection to choose from. Or at least I had, gave it to an 8 year old to start his own memories on them. My 5 favorites are these:
Black Luster Soldier Envoy Of The Beginning
A silent guardian that makes the opponent afraid even of your graveyeard, The Soldier embodied the idea of a crushing defeat while actually being fine with losing. Who doesn't wanna lose to him just to get a glimpse of the guy ?
Slifer The Sky Dragon
The one and only. Through the right cards, a deck with Slifer gives new meaning to the words 'control' and 'power'.
Vorse Raider
'Why ? It is only 100 ATK points stronger than my monster !' If only I had a penny for every time I heard someone say that. Mean, beastly and just fun to have on the field.
Crush Card Virus/Tribe Infecting Virus
I am torn cause both these 2 are excellent cards. Crush can catch one completely off guard and combines great with one of the most underused cards ever, Giant Germ. On the other hand, the TIV helped me summon the soldier easier so I decided to put both here.
And last but not least:
United We Stand
Having the potential to turn even a scapegoat into a powerhouse, United We Stand is the card that you don't expect anyone to have and shit yourself when someone does use it. The first time I actually beat BLS was with this. It also passes across a nice message: Unity. I sent my brother to pick up a Labyrinth Of Nightmare booster pack and this was inside. Favorite card in my deck and always good in a hot spot.