High Level Abilities


Priests


Elemental Transformation
Gives single-classed Druids a little melee boost, but not enough. Keep them in the back of the party to do useful things like casting Nature's Beauty or Implosion. If you want melee backup, summon an Elemental Prince. The healing when you transform back is cute but nothing compared to a decent potion. This is practically disastrous for Fighter/Druids, who should kick way more butt in their humanoid form.


Rogues


Alchemy
Yay, a single potion per day. Go steal them somewhere and save your slots for something good.

Assassination
Great ability for Fighter/Thief types (imagine the damage Kensai/Thieves could do)! Get Improved Hasted to make the most out of it. Single-classed Thieves don't have the THAC0 or attacks per round needed to make this shine.

Bards cannot choose this ability.

Avoid Death
A moderate ability practically made obsolete by Death Ward. Get a single one if you have problems keeping your Thief alive, otherwise get something better.

Enhanced Bard Song
An excellent reason to take a Bard along. The bonuses are just too good not to use. Makes the Skald's perk obsolete and negates the Blade's biggest drawback. Bard songs stack. Contemplate the effectiveness of a party of five Bards and one Fighter...

Evasion
The effects are way too short-lived to warrant filling a precious slot with this. Pick only if you want Greater Evasion.

Greater Evasion (requires Evasion)
A clear upgrade from Evasion. Get one after you're loaded with other abilities.

Magic Flute
Three Delayed Blast Fireballs per day is nice but not great. Get one after your Bard has collected enough great abilities.

The flutes last a full day but resting only takes 8 hours. Get three flutes by resting in quick succession.

Set Exploding Trap
The worst of the high level traps. Fire resistance is very common by the time you can get this. Help yourself to a Spike Trap if you like your traps to actually damage anything.

Set Spike Trap
The best trap and perhaps the most powerful weapon in the game. High damage, always hits, not stopped by saving throws or magic resistance, goes right through weapon immunities. Kill just about any creature in the game (rare exceptions noted, but nothing really dangerous) with a stack of these. Load up.

Set Time Trap
A very interesting trap that gets you a mini-Time Stop. Stopping time is good, provided you can do something useful in the time given.

Use Any Item
An insanely powerful ability, the first any rogue should pick. Ignores not only class restrictions but also alignment restrictions, race restrictions and "NPC only" restrictions (meaning you can equip items useable only by specific NPCs). Equip any armor, wield any weapon, and (probably the best of all) read any scroll.

Some tastes of what this could get you:


Warriors


Critical Strike (requires Power Attack)
A good ability if you have lots of attacks. Dual wield and get Improved Hasted to make the most out of it. You won't score many critical hits because most enemies have something of a helmet (even the mages) but at least this will guarantee that you actually hit. If you don't have access to lots of Improved Hastes you're better off with (Greater) Whirlwind Attacks.

You can pick this ability more than once.

Deathblow
Just about nothing you would hit with this would have lived otherwise anyway. Only take this if you want Greater Deathblow.

Greater Deathblow (requires Deathblow)
A surprising amount of dangerous creatures are level 12 or lower. Mow down Mind Flayers, Beholders and Abyssal Demons with this. Ranged attackers get more out of it since they don't have to waste time walking across the battlefield. Use with Fire Seeds, Arrows of Detonation or Frag Grenades from the cheesy Big Metal Rod for massive area-wide slaughter.

Greater Whirlwind Attack (requires Whirlwind Attack)
Awesome ability. Useful any time, anywhere, in both melee and ranged combat. Makes most fights a cakewalk. Unless you dual-wield and have easy access to Improved Haste you'll want to stock up on this.

Hardiness
A good defense with a decent duration. Saves you tons of punishment. Combine with Barbarian damage resistance, Armor of Faith or the Defender of Easthaven for incredible toughness. Get at least one for the toughest fights.

Power Attack
Not interesting compared to the other abilities. The average Throne of Bhaal enemy has a Save vs Death of about 5. Pick only if you want Critical Strike.

Resist Magic
Very useful if you lack the means to protect yourself from magic. Very useless otherwise.

Smite (requires Critical Strike)
Having to chase your victims usually isn't too practical, so this should be considered a "tactical use" ability for melee fighters (meaning it might come in handy in the occasional tough spot). It is great for ranged fighters; you can keep distance without even moving.

Tracking
An improved Detect Evil. A simple "life scan" isn't worth a high level ability slot.

War Cry
Worthless at this level; the save is too easy.

Whirlwind Attack
A great ability for Swashbucklers, a step on the way to Greater Whirlwind Attack for warriors.