Hecarim. Coming off a speed shrine, using E, and then running a champion down is a lot of fun. His ult is very enjoyable too. He is one of the few champions in the game whose gameplay really "feels" like it fits their character.

Playing less commonly picked champs and doing well is fun and rewarding.

Annie. There's nothing more satisfying than watching jax try to jump on your face only to hear "YAY TIBBERS!" while he's turned into a pile of molten goo.

I like to play shaco on dom, he's slippery fast and fun and I can follow around all the squishy ^.^

Syndra!

Singed + Seraph's + BFT = never turn off poison!

I will say that AD Malz bot is probably one of the most fun bot laners I've played.
Part of me thinks it's even more fun when you're dominating bot lane and the enemy sees your AD build.
Unfortunately, if you do poorly, it backfires and people assume you're trolling.

But yeah, AD Malz is a thing on Dom. Voidlings 1:1 AD Ratio OP.
That reminds me, whatever happened to Vesh? He loved AD Malz and Kat.

I also find Wukong fun. But I will admit that I have the most fun when I trick people with my Decoy.

Personally, I suggest playing your favorite champion (unless your favorite is Kass then pick your second favorite champion). Even if it is "unviable," play that champion whenever your team's composition allows. Yesterday's Trash Tier champion could very well be Tomorrow's Cheesecake Tier champion.

I enjoy Diana and Leona. Especially Leona on Dom compared to SR. Because you can actually build Leona like a damn tank, and GET ITEMS. Or, you can even do Trinity Tank Leona which is REALLY fun because people aren't expecting her to actually do damage.

Zed is my new found favorite! Nidalee and vi too. The mobility and juke possibilities make it awesome!