Right now, it's my Elec/SM Tanker. I like to be in the midst of the mobs, smacking them down, and never stopping to rest - and both my kids (who watch me play sometimes) and I smile and chuckle when he's so thick in the middle of multiple spawns we can't see him, yet the enemies just keep getting KD'd and defeated. Other than a SS/WP Brute, he's the only toon I've played straight from 1-50.
Current primary characters, all on Guardian:
The Amber Fist (Elec/Stone Tanker) | Pixelbeater (Fire/Kin Corr) | The Sequencer (Bots/Traps MM)
Blakkat (Claws/Dark Brute) | Mhogus'thra (Ill/Dark Cont) | Wyldhunt (Beast/Dark MM)
I dunno about build vs. AT here... I think I'd have to go with AT. My namesake was a stalker who got left at about 32nd level due my hemorrhaging altitis back not long after he... well, reached 32nd level. For some reason now I've gone back to him, found some fresh costume concepts and been playing him again (he's at 39th now).
Stalkers are the first AT I was playing when I finally feel I got the point of the game and how to play, so they always feel a bit like home to me. When I look at the fact that the only two 50s I have are a scrapper and another stalker, well, it all sort of makes sense.
My Mind/Electric/Cold Dom is just topping anything I have played so far.
Soft-capped, perma-Dom, perma-Hasten.
It makes me 'mwuahaha' constantly. Seriously, my buddies in voice chat are tired of hearing about him. 
"The side that is unhappy is not the side that the game was intended to make happy, or promised to make happy, or focused on making happy. The side that is unhappy is the side that is unhappy. That's all." - Arcanaville
"Surprised your guys' arteries haven't clogged with all that hatred yet." - Xzero45
Currently, my favourites are
My undestructible lvl50 DM/Inv/Soul brute. Good mixed mitigation synergy, very very survivable, and satisfying soul sucking goodness.
And my lvl50 Fire/Storm/Dark corrupter. Gives my lots of options, but for the most part he's just a rolling death machine that out damages my blasters while providing more bonuses for the team. Stealth in, Soul drain + Aim, then dual rains + fireball, all the minions are dead, most LTs are nearly dead, scourge kicks in big time, and I just blaze the rest. If there's any hard targets I can set up a storm and corner them with 'cane, then go nuts. I also have the drain for my endurance when really needed. This thing let me solo quite a few AVs/Heroes.
Celtech Main Site
My DA Profile
In before Dechs. My MFing warshade, hands down. Only my fire/ice blaster comes close to the fun of a full power tri-form.
Heh, I love these open questions... So many people answering differently.
I guess my second question (after what float's your boat?) was more around how you'd answer if someone gave you no information other than "I want to have fun."
I know that depends on their, and your definition of fun, but I rarely see that stop anyone giving their opinion anyway
Being as my last thread got 0 replies and this has now gone to page 2, I may have to draw up a list to see which is "most fun" based on the replies. Some I don't agree with, some I do, but more importantly, some I've never tried, so this could be just the thread to get me trying something new.
Thanks for all the replies!
-H
There are too many to chose from to be only one.
Fire/Psy Dominator
Elec/Elec Dominator
Ill/Rad Controller
SS/SR Brute
Fire/Mental Blaster
Cold/Ice Defender
Fire/Fire Tank
Crab Spider AoE/Ranged Build
Thugs/Traps MM
Nrg/Nrg Blaster
Elec/SS Tank
Ice/Storm Controller
Elec/Elec Blaster
The hard things I can do--- The impossible just take a little bit longer.
If numbers are so much more important than a teammate who is fun to play with, forget about the game altogether and go play with a calculator instead. -Claws and Effect-
Right now? Fire/ice blaster, bots/traps MM.
My Ice Blast/Cold Domination/Mace Mastery Corruptor. I feel truly powerful when playing this character. I control the battlefield with slows, holds and knockdowns, bring the pain with really wonderful damage, and can neuter bosses with a click.
There's a reason he's my main.
ill/kin/stone controller- (soft-capped ) flash -> FS -> fissure 
-> seismic smash followup
ice/fiery/fire dominator- (just hit perma-dom) ice slick -> embrace of fire -> RoF -> fire ball -> fire breath
bane spider solider with mace epic- 'nough said
FM/DA/soul brute- effectively unkillable (i mean, he'll just rez and stun everyone ), plus fury fueled AoE death with death shroud + FSC + dark obliteration.
50: Ill/Kin(A+,R,J)-1047 badges RE/Dark(A) Fire/Elec Warshade BS/Regen Necro/Poison Ice/Fiery(A+) Son/Son Bane(A) FM/DA(A) DM/Nin Grav/Icy
lvling: Inv/EM DM/Sheild Arch/MM Bane NW Elec/Earth Grav/Elec Elec/FA Rad/Ice
Paragon Elite/Rogue Elite Joined Oct 2004
Thinking about this made me realize how well balanced the game is. There really are a lot of answers, depending on your priorities. I think an overall generic answer should be able to team and solo uber well, plus have a smooth leveling experience. Even then, only a few AT's do poorly (most Tanks, Controllers, Defenders). Not that they can't solo easily, but not "uber" (notable exceptions). Some builds can do stupid well solo, but don't really participate in team dynamics. Some are uber endgame, but painful to level. Or they are gods early, but bleh endgame.
So my list includes AoE Brutes, particularly Willpower. Brutes and Willpower are great early, and AoE plus survivability makes them solid endgame players. Pretty much all Doms, though I'll single out Plant/ for smooth leveling while still being an endgame god. One gives you smashtastic in-your-face fun, the other is probably the most cackletastic. If you wanna take a beating and give it back twice as hard, Brute. If you want to destroy vast swaths of bad guys in a variety of ways while never really feeling threatened, Dom.
Cloaking Device: check.
Targeting Drone: check.
Now, I admire the long string of Trip Mines I placed perfectly along the line which I know the mobs will follow.
Caltrops down at the point where they will obviously round the corner.
And might as well call the Gun Drone to mop the ones that are a bit resistant to hot lead.
Windage: check.
Arc: check.
Control breathing. Squeeze the trigger.
Oh, to send a round with laser precision into a mob's head (from more than a few yards away)... Then wait and watch as a perfectly laid trap mops up his buddies as they race toward my hiding spot to see what dropped their friend...
This is what makes me happy. To be a perfect assassin, a soldier of fortune. A master of ambush. Perhaps I cannot take a beating if they get close. So, I don't let them get close.
My happiness build is my Assault Rifle/Devices Blaster. No matter how many builds I try, and delete, I always come back to SSG Jones for precise, planned attacks.
Why a Harley? If I had to explain, you wouldn't understand...
Currently I am really enjoying my Spines/Invulnerability Scrapper. Lots of AoE, capped S/L and E/N Defenses, a self heal, debuffs and immobilizes (with the dark epic) and enough good looks to make me jealous on the Dopleganger Tips mission.
But since I am an altaholic, it hard for me to suggest any one AT or power combination. There are so many good sets that it is hard to say what may work for someone else. And for me, it is hard to spend too much time on a single character.
But here are some others that make me smile every time I play them:
Ill/Storm Controller (this build is pure chaos <3)
Fire/Kin Controller
Katana/Regen Scrapper
SS/Willpower Brute (this build is non-stop SMASH!)
Fire/Dark Corruptor
Fire/MM Blaster
Tri-form Peacebringer
50 Fire/Kin Cont
50 Fire/Axe Tank
50 Spine/Inv Scrap
50 Eng/Dev Blast
50 Claw/SR Scrap
50 Emp/Dark Def
50 Eng/Elec Brute
50 Fire/MM Blast
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