Ground and rock types are usually a safe bet. They're not great around water, but for tunnels and mountainsides, you can't really go wrong.

[Hmm, but which ones are the most feasible?]

I'd say Onix is probably the safest bet in that it's huge, rideable, and evolves into a giant metal snake if you need something that's equal parts intimidating and sturdy.

[He chuckles.]

But I could probably get a more comprehensive list if you wanted more options. Juggled enough rock, ground, and steel types to get a good read on what works and what doesn't.