Extraordinary healing to full in 1 round, every round
You mention Tome of Battle, but note concerns with recovery. I note that stances don’t need recovery, and martial spirit conveniently lacks rules about the attacks being against “threats” or “opponents” or “aggressive creatures” or whatever:
While you are in this stance, you or an ally within 30 feet heals 2 points of damage each time you make a successful melee attack. This healing represents the vigor, drive, and toughness you inspire in others. Your connection to the divine causes such inspiration to have a real, tangible effect on your allies? health.
Each time you hit an opponent in melee, you can choose a different recipient within range to receive this healing.
That’s 2 hp/attack, and you can keep it or give it to a friend.¹ More attacks you have, more hp you can heal. And best of all, it just says “successful melee attack”—absolutely nothing about an enemy, a threat, or anything else. So it probably works best as some kind of barbarian/chaos monk with Two-weapon Fighting² whaling on a training dummy or something (could be a brick wall or the ground for all the rules seem to care) maximizes our results, but ultimately it isn’t terribly important. A decent 20th-level barbarian has maybe 336 hp during a rage,³ so you can fully heal up about the beefiest target available in just over two minutes.⁴
But, of course, that’s just one feat,⁵ a single 1st-level stance. We haven’t even finished getting through Devoted Spirit yet, because your single best bet here is probably Tome of Battle’s aura of triumph:
While you are in this stance, you and any ally within 10 feet of you both heal 4 points of damage with each successful melee attack either of you makes against an evil target.
Note that it requires an evil target, not necessarily a threatening one. So while you can’t attack the ground itself,⁶ you could attack, say, an evil item. Of which there are many, whether we’re talking about cursed objects or just stuff like unholy weapons or whatever. And attacking objects is explicitly a thing in the rules,⁷ and they are “targets” of your attacks, so, cool. Better yet, magic items are damnably⁸ difficult to destroy, so you’ll be able to make successful melee attacks against that item for a long time before you have to worry about replacing it.
Anyway, access is somewhat less trivial than just “take Martial Stance any time you like,” but warblade has only extraordinary class features,⁹ so it’s easy enough to take levels of that for the sake of reaching initiator level 11th. Then just pick up Martial Study (crusader’s strike), Martial Stance (martial spirit), and Martial Stance (aura of triumph) at your convenience. You can have all 3 by 12th level if you want.
Non-warblade levels, as usual, count half towards meeting the initiator level 11th requirement on aura of triumph, so up to two of them are “free” for this exercise (they won’t delay aura of triumph since you don’t get a feat at 11th anyway so you get it at 12th either way). And of course, delaying things would be allowed and we could always pick up aura of triumph at 15th or 18th level, or even at 20th if that level was fighter, or got a fighter bonus feat. The bare minimum number of warblade levels here is 2, though that forces a level of fighter at 20th. To hit IL 11th by 18th, you need warblade 3rd.
Probably the ideal thing to do with the rest of the levels is, again, to rack up extra attacks. The “melee” requirement actually winds up being kind of meaningful, because ranged attacks are way easier to come by, plus throwing weapons could combine those with two-weapon fighting for the best of both worlds. But anyway, still plenty of options for melee attacks too. Maybe pick up avalanche of blades along the way, and grab Stormguard Warrior so you can make all of those attacks as touch attacks—and deal no damage. Boom, you might get 100 hp out of that every other round.
But ultimately, that optimization is probably wasted—because that’s not the real gold here. The real advantage of aura of triumph is that your allies’ attacks count too. They can’t heal each other, but they can all heal you (and themselves at the same time). So really it’s more like, everybody get in a huddle and kick this evil monolith, and you’ll heal me to full in a round, easy.
Switching who you’re healing, amusingly, might require an actual “opponent,” due to the last line of the stance.
Or just “two-weapon fighting,” that is, without the feats, since you don’t really need to care very much about your attack bonus when your target can literally be the broad side of a barn. Since city-brawler barbarians get both Improved Unarmed Strike and Two-Weapon Fighting (for unarmed strikes), though, at basically zero cost, you’ll probably have the feat, and it allows for yet more attacks as BAB increases and you take ITWF and GTWF.
Assumed average HD rolls and 30 Con, which might be low if you’re actually a single-classed barbarian and have mighty rage, but realistically almost no one does that so it’s probably just rage and for rage it might be a bit high, so I call it a wash and say it’s a reasonable ballpark figure.
I assumed full BAB, whirling frenzy, and the Two-weapon Fighting line, for 8 attacks per round, for 16 hp/round, for (exactly!) 21 rounds to heal 336 hp. That’s 2 minutes, 6 seconds. As noted, with chaos monk or whatever you can do better than 8 attacks per round, but that math gets uglier and it’s kind of besides the point.
Well, one feat doing the healing; a bunch of class levels and feats getting the attacks you want to fuel it.
Most places, though if you find yourself in the Lower Planes you should totally give it a try.
Sundering; it’s plausible for people to forget it’s a thing players can actually do since it’s so rare that they’d ever want to, but you can.
I’m not even sorry at this point.
There are some really dubious extraordinary abilities among warblade maneuvers—earthstrike quake is my personal choice for greatest offender—but if we were worrying about something like that we couldn’t use martial spirit or aura of triumph to begin with.