I'd have to say my favorite new pre-E4 evolution is Honchkrow. I think that thing is amazing, when you think about it, Dark/Flying is a very good typing (both of dark's weaknesses are made neutral leaving it with only your basic flying weaknesses to rock and electric). post-E4 is a tough call, I'd say either Mamoswine, Yanmega, or Magnezone. I'd say Magmortar but i think its a bit overhyped.
the thread starter made some good points. I think Mawile especially has tremendous potential for an evolution, perhaps even a dual evolution (one keeping its pure Steel type... and another that is dual Steel/Dark because Mawile is a deciever... the Dark type fits it)... Houndoom is also one of my favorite pokemon but i think it is fine as it is, its design pretty much ensures that if it gets any more "demonic" GF will get sued or something... i can't really imagine an evolution for it. its kinda like a good movie... sometimes you shouldn't make a sequel.
as for ideas... Mawile, definately... Pinsir could use an evo, perhaps a Bug/Dark or Bug/Fighting combo. A Rock, Steel, or Dragon Eeveelution is in order. I think an alternate Vulpix/Growlithe evo would be cool, kinda like what they did with Poliwhirl to Poliwrath or Politoed. Vulpix could have a fire/ghost evo and Growlithe could have a pure Fire or Fire/Dark evo.
Sandslash needs a Ground/Dark or Ground/Poison evo desperately.
A third Nido would be awesome.

perhaps a Poison/Dragon!
from Johto... Ariados... definately.... its a spider that can spit sludge/shadow balls and use psychic... it deserves an evo and its a crime to keep it as underpowered as it is.
Qwilfish should become a hydra... i don't know how but they should do it. if GF could make a remora into an octopus, they can make Qwilfish into a Hyrda. maybe change its secondary type to Fire???
Magcargo. No type change... just give it an upgrade and some seriously better stats. it should become a big lava monster or something.
from hoenn... Surskit needs an alternate evo. the loss of the Bug/Water typing in Masquerain is a crime, if Surskit got a Bug/Water upgrade... it could put its awesome typing to use (Bug/Water elminates bug's fire weakness and water's grass weakness.
A Torkoal evo is in order. perhaps another addition to the Fire/Steel camp?
Absol is pretty awesome as it is, but it could still have an evolution...
Huntail needs to have a Water/Dark evo, and Gorebyss needs to have a Water/Psychic evo.
not much is lacking in Sinnoh considering that out of the 106 new pokemon, 53 (thats half) are starters, fossils, new evos, or legends. if there is one that needs an evo, its Rotom, because i love the thing, its typing+its ability are awesome it just needs better stats.