When you phrase your question like this (and I think this is the proper question), I think the answer is even more clear: SK & druid (or shaman). A real tank is the only way to ensure that you can beat as much of the content as possible. If you don't want to play a druid (which grants double-invis with the SK, decent damage and buffs, tracking, teleporting for fast travel and more), then you could honestly pair just about anything with the SK.
I think the druid has more general utility and quality of life abilities (like transportation - which is a huge time saver). Druids have some useful mob debuffs, but not as nice as the shammy's Slow. I think Druids have better synergy with casters (SK's cast and melee). I think the shaman has better melee synergy (and can slow mobs) and will boost the SK's power in combat more than the druid will. DPS-wise, they are similar, but I think the Druid has the edge due to more nukes (better for shorter fights like most group content). Also, Druids can heal the tank with some of their nukes later on, which lets them do more DPS while also making your tank tougher (even with healer mercs). They are close and either will do fine. I prefer the Druid (druid happens to be my raiding main and my first toon, so I'm biased ), but I have also paired with a shaman (during classic and Kunark on a TLP server) and they really are great also.
If you have a good tank then there really isnt much call for using pulling skills. But sometimes you dont have ideal groups. Then it makes sense to bring singles.
