Which skillset do you think would be the best? (with only one R skill)

I'ld go for
Tristanas 90% attack speed sterioid
Bio arcane barrage
Kayle's righteous fury
Spray and pray

Another good one:
Turrets
Boxes
Traps
Shrooms

For the most hilarious unbalanced lolz:

Blitzcrank's Rocket Grab
Alistar's Pulverize
Rammus's Puncturing Taunt
Skarner's Impale

or substitute Cho'Gath's Feast because eating people is fun.

Alistar with evelynn's stealth. I suppose remove his heal. I mean think about it, stealth pulverize headbutt win.

Dazzle
Cryptic Gaze
Rupture
Crescendo

High CD, Stunstunstunstunstunstunstunstunstunstunstun

Annie's passive, Rammus speed ball,Sions Stun, Vlad's pool, fiddle sticks ultimate. Think about it.

Poppy's % damage
Vayne's Silver bolts
Kog's % damage ability
Cho'Gath's feast.

Tanks nightmare.

Vayne's Silver Bolts
Kog's % damage
Trist's AS steroid
Sion's Ult.

Rush Phage. They will never escape.

Evelyn's Shadow Walk
Alistar's Headbutt
Morgana's Black Shield
Nunu's Absolute Zero

Cast shadow walk, while stealthing cast absolute zero; proceed to cast absolute zero while stealthed.

Of course this basically is a walking ultimate the rest of the time you can go around stealth headbutting people

Soraka's heal
Sona's heal
Soraka's ult
Taric's heal

tristanas passive, kogmaws w, Tristanas attack speed buff, Kayle's righteous fury, Tryndamere's ultimate,

Insane range + unkillable, + attack speed.

=)