If you don't mind me asking, would your opinion on the Shinx family change if Luxray were given a Dark secondary type? This is something I frequently see people discussing, and I was curious as to what another Shinx/Luxio/Luxray fan would think :)
I think that appearance-wise, Luxray fits the type in how menacing it looks (particularly its eyes) and Crunch STAB would be an intriguing addition to its arsenal, but I don't know if it'd be worth adding another common weakness (Fighting), particularly when it's fairly slow for an offensive Pokemon. The added resistances may be intriguing for in-game use, though.
The one that surprised me the most, however, was Electrode. I'm using one in a challenge in Sapphire, and I wasn't really expecting much from it just from looking at its Bulbapedia page - it's blisteringly fast, yes, but the rest of its stats were mediocre at best, and it didn't really have access to any decent offensive, non-Electric moves. However, ever since I've picked one up, it's been doing relatively well on my team. Because of its speed, it works well as a support member, with mine having Light Screen and Screech, allowing it to set up on or cripple anything it can't hit with Thunderbolt.
To sum it up, my favourite was certainly Galvantula, but really, they were all pretty interesting to use. If anything, the only one I've found 'disappointing' in any way was Electrike/Manectric, but that's only because I used one in Gen III, where the only way to get Crunch for coverage was to breed it in (which I was too lazy to do at the time, haha). It did well in my team, but it didn't have any support options outside of Thunder Wave, which isn't as effective as what Electrode has going since whatever Manectric can't hit with Thunderbolt, it can't hit with Thunder Wave :P
I disagree with what I underlined! Look at how cute it is--
Image source: click here!