Bad title, should have said "Favorite", not "Best". Anyway.

Maokai is an obvious choice. ButI have to say, playing amumu this week has really rocked my socks. Singed is, eh, he's okay, so is taric. But Amumu rocks it. And personally, I'm still hoping to make Galio work when I get around to buying him.

Maokai
Singed
Alistar
Rammus

I usually play Maokai or Leona. I've been trying to pick up Blitz lately.

Maokai's a definite favorite of mine since I got to play him the other week when he was free, though the only tank I enjoy that I own is Alistar. Actually had a game last night with him with a Poppy and Vayne; it was pretty humorous when the three of us ran around together. Headbutt+Heroic Charge+Condemn. Once they were against that wall, they never left it. xD I think I actually pushed someone into the terrain at one point with all the knockback spam.

rammus
alistar
malphite

She is situational, not worth 6300 unless you REALLY wanna play her. She depends heavily on her team just like in SR.
Leona can amplify your team's worth, but if they are **** then theres not much she can do.

10 * 0 is still 0

She's really good, sometimes. She's very team dependent (you won't be dueling people down unless they're squishy and maybe half health), so I usually only play her when I queue with friends. Maokai is a much safer solo queue pick, so I end up playing him most of the time. They're both a lot of fun.

Leona is pretty decent if you're already good with her. The only reason I would recommend against her is that you'd probably rather have blitz.

His base stats outrank hers: more hp, ad, armor, mr.
His Q beats her E in almost all cases on Dom since there are few minions to get in the way, even though her CD is much lower.
Her Q is about even with his E.
Her W shield+her passive damage is slightly more tanky, while blitz' MS/AS buff W + his shield passive is slightly more damage and much more movement.
Her Ult is a bit nicer, but he gets his twice as often and it's easier to hit with (his does more base damage, but he probably won't be stacking AP so hers will actually hit harder).

She can certainly defend points better. Initiation is a toss-up since she has more stuns & can take more hits. He can do a lot more damage and get to where the action is a lot more quickly. She could certainly hold bot better if your bot needs to switch. Blitz is pretty useless bot.

So she's certainly viable, especially if you're premade & you can coordinate her passive, but she's a bit underclassed. If you're looking for someone to hold a point & stun, Ali beats here there.

I've never really been glad to have a Leona on my team. Maybe I've only been with bad ones, but in every case I'd have preferred she had picked a different tank, or just a DPS.

They never seem disruptive enough to really help out, and they deal almost no damage.

I believe any champ can be good in Dom in the right hands, but I think she's one of the ones like usually just isn't really worth taking over pretty much anyone else.