I love Famine in Guild Wars. It’s on every eligible team. Even as the number of troops immune to mana drain increases, very few teams are totally immune to it. The ability to suck mana from stealthy targets, and deal one heck of a punch while doing so, make it one of my favorites. It doesn’t matter how much damage an enemy spell would deal if you can prevent them from ever casting it!
Ubastet has the exceedingly rare instakill ability. Sure, you need to be a bit careful when you fire him off, but that applies to most spells in the game. And unlike Wulfgarok, who devours on a kill, Ubastet’s spell scales with enemy stats. Plus it uses red and yellow, traditionally the colors with the best mana producers. Arguably Irongut has the same instakill power, but (a) as many troops are immune to devour as to mana drain, and (b) it’s a lot easier to deal damage than to boost attack in GoW.
TPK’s third trait is so, so good. If you can maintain a gemstorm somehow, you will darn-near constantly be cycling through the board, since explosions are likely to set up cascades that turn into additional 4+ matches. Even if not, he can boost your mana production considerably, and without penalty: his third trait only ever fires on a 4+ match, which means you never have to worry about turning over a good board to the AI. His spell is useful as a panic button, but I almost never cast it. TPK is a Delve staple for me.
Edit: another way to describe TPK. Any time you get an extra turn, gain bonus mana. Given how often you want to make 4+ matches during a battle, that really adds up!