decks I always have on hand are

TNB which I keep updating
Hellboy/Underworld haven't changed since I built it
Insanity which I keep Modern (well until DWF rotates out)
KiwiFruit the deck we played at Sydney 10k which went 9-0 against Light show
Jean Grey Legend

Then the rest keep getting built and pulled apart depending on tournament Format.

I always have

Brotherhood/Arkham
Legionnaires
Spider/Defender

Brotherhood
MKO
... and some deck with Batman in it.

New and Improved Du Jour
Wolverine Legend
IG Hand Punishment
GK Variant(Currently Robber Barons)

Always have those built and ready to roll

Avengers (I just updated it to include the MUN pieces that I didn't already have tied up in my modern age Avengers deck)

Underworld/Defenders

Gotham Knights (also constantly updating with new sets and such)

Basically its plug n' play which Kang you like at that drop until 4 when you play Master of Time to draw the stuffing out of your deck. Spud's build has a little too many characters for my preference because I like to run some of the more annoying Kang stuff like; Time Theif, Game of the Galaxy, Null Time Zone, Spheres of Solitude, along with a few of the Gems.

My Mainstays
GK/JLA Trinity, (Supes, WW, Bats)
Skrull/Inhumans
JSA Up Curve/Rock Deck
Defenders
Darkseid's Rock Deck

Right now, however, I'm working on various MUN teams, T-Bolts, CrimeLords and Avengers.

similar to captainspud, I keep 30-40 decks running and protectored up (though he has > 50. nice!). Too many so I won't list them all here.

Man, I'm fickle. If I play a deck longer than a year, it's a rarity.

The oldest deck in my case is Villains United. Even without getting any further support, they were just that good out of the gate. I don't play them that often. But, if my win-loss recored is lousy for the day, I break 'em out. Even if I loose with VU, it's usually cathartic :)

My Darkseid deck changes every time I play it. I'm always tweaking it a little. It's a delicate balance of combat stuff and resource row hate. Not to mention substitution love. I almost retired it a while back. But much to the chagrin of my playgroup, DC Legends had just enough DE love to keep me interested.

I love my Revenge Squad deck. World's Finest gave me all the tools I had been hoping for since Man of Steel. It's not overpowering, but it sure is fun to play. And every time a new non-team-stamped on-going plot twist comes out, I have a new tool to play with.

Also from World's Finest, I play the heck out of Birds of Prey. No team-up needed. Just lots of concealed attackers with plenty of attack pumps. And the ability to put a big butt in the visible area when I need a defender.

I've got some decks built with older teams, but they have all been built within the last year. These are the only decks I think I've played with any consistency for 12 months or more that I still play today.

Before I got ride of my cards my only mainstays were Xavier's dream and Curve Sentinels. I need to build both of those on MWS now that I think about it.

I play at least once a week so I like to have a bunch of decks accessible to me...

Right now I have constructed;
-Illuminati [Modern] <-- Current Favorite!
-S.H.I.E.L.D. Army [Modern]

-Marvel Defenders - HULK SMASH! [Silver]
-Fatal Five - Recover & Burn [Silver]

-Birds of Prey/Gotham Knights [Golden]
-Wild Faces of Evil (Faces of Evil with Wild Pack) [Golden]
-Punisher/ Marvel Knight - Equipment [Golden]
-Avengers - Reservist [Golden]
-X-Statix Loner [Golden]

Plus I have a few piles of cards which can easily be turned into decks;
-Insanity [Modern, Silver, or Golden]
-MKKO [Modern, Silver, or Golden]
-Squadron Supreme - No Hand
-BPRD and/or Hellboy Abuse
-Thule Society and/or Project Ragna Rok Abuse
-Arkhan Inmate - Random Discard
-Avengers - Leader/Team Attack

#1 Avenger Reservist (Black Widow QuickSilver, Warbird)
#2 Hell Fire Club
#3 IG burn
I have other decks built - X-men energy burn, X-men weenie team attack, Kree, Hog, and others. But the 3 above are always ready!!!

Hey spud, got a deck-list for this one? Ever since I played against this deck long ago...I wanted to piece one together. Thanks!

I've got over 35 decks that I have put together...

Arkham Inmates - Random Discard
Arkham Inmates - Exhaust/KO
Arkham Inmates - Non-Adjacent
Arkham Inmates - Insanity
Arkham Inmates/League of Assassins Team-Up
Darksied's Elite - Female Furies
Darksied's Elite - Location Burn
Darksied's Elite - Darksied Clones
Darksied's Elite - Face-Down Resources
Darksied's Elite/Deathstroke Team-Up
Injustice Gang - Hand Fill
Injustice Gang - Army Exhaustion
Injustice Gang - Joker/Luthor Legends
Injustice Gang - Crime Syndicate
Injustice Gang/Anti-Matter Team-Up
JLA - Plot Twist Control
JLA - Willpower
JLA - Kyle Rayner/Steel/Aquaman
JLA - Power-Up Equipment
JLA/JSA Team-Up
Revenge Squad - Front Row Swarm
Revenge Squad - Breakthrough
Revenge Squad - Ongoing Plot Twists
Revenge Squad - Bizarro World
Revenge Squad/Villains United Team-Up
Secret Society - Leader Curve
Secret Society - Character KO
Secret Society - KO Pile
Secret Society - Character Steal
Secret Society/Doom Patrol Team-Up
Team Superman - Superman Clones
Team Superman - Protection
Team Superman - High Defense Stall
Team Superman - Invulnerability
Team Superman/Checkmate Team-Up
Fearsome Five/Secret Six Team-Up
Villains United - Character Return

Once I get a deck where I want it I rarely touch it except for maybe some small tweaks with metagame cards. Here is a list of my decks:

Good Guys:
Net deck with small tweaks. I built it a few months after I started playing and it was the first deck I built that was actually any good.

Morlocks evasion:
First deck I built myself. Well, I didn't do it in a vacuum. I started with morlocks physical and then saw someone at hobby league with an evasion version. I very quickly tore the physical apart and built the evasion. It is still one of my favorites and totally wrecks GA Avengers reservist.

Golden Thunderbolts:
Saw a Thunderbolts team tactics build posted and decided to give it a try. I found team tactis to be a little situational so created my own version focusing on mechbay.

Doom Moloids:
First deck I built in a vacuum. This is one of my very favorite decks simply because it is all me.

X-Statix:
This is one of my favorites. I love the team. Fun to play. Not much more to say.

Squadron No Hand:
Netdeck. I bought all the cards online for under $20. Competitive and fun to play.

Spider evasion:
Net deck with minor tweaks bought online for under $20.