You need to give more info. What decks do you have? Do you have any strategies you want or don't want to play?
My favorite all-time deck is Karador, Ghost Chieftain. I play a control/stax build that tries to answer every possibility.
My most fun deck is Maelstrom Wanderer. People say that you can't build him wrong, and I agree. MW lends himself to many different types of builds. He is a value engine and win-con wrapped into one. I made the most aggressive build I could make to try to end multiplayer games as fast as possible.
Also most of the preconstructed decks are only a few key cards away from being good, as long as you can separate out the parts meant for the alternative generals.
Try searching for legendary creatures in magiccards.info and see if there are any you like, and we can give advice on what sort of decks to build for that general.
Honestly just pick up one of the precons and build upon it. Some of the best generals come from those, particularly the 2011/2013 ones. They're easy yo upgrade, flexible, and potentially A-S tiers in some cases
Mono Green Mill using Omnath as general
Mono Blue aggro (non merfolk)
or pending your play group any of the nephalim
My favorite all-time deck is Karador, Ghost Chieftain. I play a control/stax build that tries to answer every possibility.
My most fun deck is Maelstrom Wanderer. People say that you can't build him wrong, and I agree. MW lends himself to many different types of builds. He is a value engine and win-con wrapped into one. I made the most aggressive build I could make to try to end multiplayer games as fast as possible.
Zedruu the Greathearted is a lot of fun, but also very different.
Mono-black Control is also fun, and there are a lot of generals that you can use.
8.RG Green Devotion Ramp/Combo 9.UR Draw Triggers 10.WUR Group stalling 11.WUR Voltron Spellslinger 12.WB Sacrificial Shenanigans
13.BR Creatureless Panharmonicon 14.BR Pingers and Eldrazi 15.URG Untapped Cascading
16.Reyhan, last of the Abzan's WUBG +1/+1 Counter Craziness 17.WUBRG Dragons aka Why did I make this?
Building: The Gitrog Monster lands, Glissa the Traitor stax, Muldrotha, the Gravetide Planeswalker Combo, Kydele, Chosen of Kruphix + Sidar Kondo of Jamuraa Clues, and Tribal Scarecrow Planeswalkers
Jarad, golgari lichlord, is super easy to make and can easily table wipe.
Prime speaker Zegana, play large creatures draw cards what more could you want?
Larzav, Dimir Mastermind, what is yours is mine.
Also most of the preconstructed decks are only a few key cards away from being good, as long as you can separate out the parts meant for the alternative generals.
Pioneer:UR Pheonix
Modern:U Mono U Tron
EDH
GB Glissa, the traitor: Army of Cans
UW Dragonlord Ojutai: Dragonlord NOjutai
UWGDerevi, Empyrial Tactician "you cannot fight the storm"
R Zirilan of the claw. The solution to every problem is dragons
UB Etrata, the Silencer Cloning assassination
Peasant cube: Cards I own
Perhaps more helpfully I would always suggest one of the commanders in my sig.
BRGKresh the BloodbraidedBRG, A box of lands and ideas.
Modern:
RG Titanshift. A deck made of cards too stupid for EDH.
Retired: Lots. More than I feel you should suffer through or I should type out.
Playtesting | Karador, Ghost Chieftain | Narset, Enlightened Master | Ephara, God of the Polis
Established | Gahiji, Honored One | Shirei, Shizo's Caretaker | Opal-Eye, Konda's Yojimbo | Rubinia Soulsinger
Retired | Medomai the Ageless | Diaochan, Artful Beauty
Ghoulcaller Gisa
Feldon of the Third Path
Karametra, God of Harvests
Brago, King Eternal
Running off ideas...
Make a three- or five-color EDH.
Complete a color wheel with a mono green, blue, or white deck.
Pick an archetype you haven't ( I'm assuming ) done yet. Something like tokens, or stax, or voltron.
Make a hyper-competitive Azami, Lady of Scrolls deck and win a lot and have everyone hate you because you play blue.
Make a hyper-competitive Azusa, Lost But Seeking deck and win a lot and have everyone love you because you play green.
Make a turbo fog, pillowfort, and counterspells deck and see how impossibly long you can make a game.
Make Heartless Hidetsugu deck and kill the whole table, including yourself, on turn five.
Have fun!
UTeferi, Temporal ArchmageU's prison: blue is the new orange is the new black.
BSidisi, Undead VizierB: Bis zum Erbrechen
GTitiania, Protector Of ArgothG: Protecting Argoth, by blowing it up!
GYisan, The Wanderer BardG: Gradus Ad Elfball.
Duel EDH: Yisan & Titania.
In Progress: Grand Arbiter Augustin IV duel; Grenzo, Dungeon Warden Doomsday.