High floor, high ceiling

[G.Cube]

Are there any classes out there that would fit into this criteria?

[Ravens_cry]

Druid. Druid. Druid. Druid.

[nedz]

Druid. Druid. Druid. Druid.

I'll have a parrot as my animal companion, and I'll Wild Shape into a Donkey.
Now I didn't take Natural Spell so no spell casting for me, besides I only prepared combat healing spells.

I also took the Spontaneous Rejuvenation (PHB 2, p 39) ACF so that I can heal people instead of haveing Spontaneous SNAs.

[Lans]

Beguilar, Summoner, and Dread Necromancer depending on how high is high.

[Red Fel]

Druid or Cleric, in my opinion. It's possible to mess up the character, but you really have to work at it.

Ignoring ACFs, variants and PrCs, the worst thing you can have with a Cleric is a capable melee combatant with a few useless spells. The worst thing you can have with a Druid is a spellcaster with some useless spells who can turn into a useful (if not optimized) animal or two. You really have to work to break them.

Wizard is easy to break, if for no other reason than they have so little without a good spell selection. Sorcerer moreso. But Druid and Cleric still have options. Particularly Druid. A Druid can always SNA. A Druid can always Wild Shape. A Cleric can always smash a face and wear heavy armor.

Yes, there are ACFs that can get rid of some of these, and multiclassing can rob you of some options. But if we take all ACFs and variants into account, nothing can have a high floor (apart from ToB), because everything has been given cruddy options (except for ToB) over the life of 3.5.

One villain's two bits.

[eggynack]

I'll have a parrot as my animal companion, and I'll Wild Shape into a Donkey.
Now I didn't take Natural Spell so no spell casting for me, besides I only prepared combat healing spells.
Well, yeah, but you can do that for anything. Also, that's a gameplay floor rather than an optimization floor, I think. Hand me your 11th level druid who is wild shape'd into a donkey, lacks natural spell, and has only prepared detect poison in all of their slots, and it will only be a standard action until I've turned back into my normal form, and then another round until I've spontaneously converted one of the 6th level detect poisons into an oread and used it to make an earthquake. And then, when I'm done spontaneously summoning, I turn into some massive creature of wide renown (probably a dinosaur, or other beatstick with reasonable AC), and consume people's faces.

After that, it's only a day until the parrot has become a second fleshraker, and until my spells have been replaced with good spells. Because problems don't stick around. So, yeah, this will work fine. Doing the actual build optimization for a mid-op is as easy as trading one of those feats that aren't natural spell for one that is natural spell, and I can break down the high ceiling for druids if that's also debatable. I'd take this no-op druid over a no-op fighter, or even a mid-op fighter, because it's only a couple of rounds until I've turned the no-op druid into a reasonable druid, and a couple of days until I've turned the reasonable druid into a pretty great druid.

Edit: Spontaneous rejuvenation does hurt a bit, but I wouldn't characterize using crappy ACF's as a function of floor. That's actively acting against the character's power, which isn't something I'd expect out of someone new to the game. Also, even a druid who has traded away everything from the animal companion to wild shape to summoning is still only a day away from having good spells, so that part still works.

[Ravens_cry]

Druid or Cleric, in my opinion. It's possible to mess up the character, but you really have to work at it.

I'd say Cleric is lower floor than a Druid, as a heal-bot cleric can work with a lower-op group without overshadowing.