Now that I think about it, I do like a lot of unloved, forgotten or outclassed Pokemon. I enjoyed using Maractus because, despite its only good stat being Special Attack, it was really fun to use- plus its design is actually kind of adorable to me. It's a personal favourite of mine because of all that.
Compared to others of their typing, I don't see Sigilyph or Octillery getting much attention. Honestly, though, even if they aren't quite as strong, they have some pretty good coverage. I know for a fact that Sigilyph can learn Ice Beam and Flash Cannon by TM (which kind of amazed me when I learnt it), plus the typical Shadow Ball, Psychic and Air Slash. Octillery can learn Ice Beam (I think) and Signal Beam by level-up, and both Flamethrower and Psychic from TM at least. There's probably more, too. Honestly, both Pokemon become pretty reliable members once I get them because they can destroy so much. It's too bad that Gen 6 ruined Sigilyph's cry, in my opinion.
Marowak I've already explained.
Flareon and Glaceon may be considered two of the worst Eeveelutions, but honestly, that most certainly does not mean they're bad in general- especially not in Gen 5 and beyond. Flareon can become a real force to be reckoned with in Gen 5 with some TM use (Return, Dig, Work Up and Fire Fang did a lot to help it along), plus, it's surprisingly resilient, and Glaceon, though its learnset isn't too spectacular, is a lot tankier than you'd expect, especially for an Ice-type. TMs can fix that learnset as well- Shadow Ball takes advantage of its high SpA stat, and even without STAB, it can do quite a lot until you can get your hands on Ice Beam. It learns Icy Wind relatively early, too, so provided that you can evolve it before it does, it can carry Glaceon quite far.
I don't see Hydreigon's line getting much love either. A shame, really- they're my personal favourite Dragon-types. Even with Hustle.