Everything has it's uses.
But as a transmuter, a LOT of people will want to take you places so it's real easy to get friends. The down side is that if you explore flat you get observation xp, but virtually no exploration xp.
Back in the day with Quesnel, my first true exploration character, he was a bard. I did most of it solo too. I explored so much of the mud that when they did an easter egg hunt it was all fresh on my mind and I found 26 eggs where the next guy up only had like six or eight.
Believe it or not, I like exploring with druids because they are so versatile. With the proper herbset + moonblessed edge and the right mooncycle, they can outheal a healer. No sanctuary though.
But if you're thinking side kick, then bards and trannies are easily the best choices. Invokers are niche things that make getting certain items and places much easier. But there's not a lot of places you really NEED one to get to.
I'm loathe to play any shapeshifters. I haven't been able to play one since they were introduced to the mud. I was really pissed that my transmuter lost prismatic spray only to get it replaced by a god damn boar... though hold person + boar + transform = dead villager