What's the best type of twin bed for a toddler transitioning from a crib?

Any kind will work.
One with a trundle bed underneath can be useful for sleep overs... however, he is a boy and they don't have many of those. One with drawers underneath can be great as the years go by... drawers are always handy. One that's open underneath gives him a place to stash things - large pieces of plywood with train layouts on it, for example.

If it is a taller bed, make sure you have a guard rail to start and a step stool to help him get in and out if he needs it.

we are going through the same thing and right now we have a bed with rails that i think is a little too high up for my son, even though he seems to have no trouble getting in or out of it. i'm mostly concerned with him jumping and playing on the bed and (God forbid) falling off. i think having a platform bed, since its closer to the floor, is a better idea, at least for us.

we have a twin and box bring on a steel frame, no head board or anything yet. We put her in it at 2, she fell out only once.

My son slowly transitioned to his big bed slowly, starting around 18 months. First we put his crib mattress on the floor. Then we brought his twin mattress home and put it on the floor. We did both for about 4 days each. It was a very exciting process for us. Eventually we put his mattress on his captain bed and for the first few weeks the couch cushions on the floor beside his bed. lol
My daughter unfortunately broke her crib, she was a very strong 19 months! lol So we moved her to a toddler bed with rails at the top. She is a bit of a clumbsy girl and falls out of her bed occasionally but it's a short drop!