1. Iron sleeves (replaceable) in aluminum is nice, but costly, and can introduce sealing problems. These can be a tight press fit, or a looser fit with o-rings, and with water circulating against the iron ("wet" sleeves).

2. Best for heat transfer and minimum weight, and very good for durability (now that the bugs have been worked out) is aluminum with the surface etched to leave a silicon rich area that is quite hard and wear resistant.

3. Hard chrome plated aluminum works well too, but is is expensive and cannot easily be rebored.

3. Cast iron is hard to beat for cost, and it is easy to rebore.

It depends what you mean by best....

The absolute best is beryllium - lightest and has an affinity to absorb great amounts of heat. (used in spacecraft, etc)

The cheapest and next lightest is aluminum - 3rd most abundant element in the earths crust, however oxidizes easily and reacts violently with oxidizers

The next best is Titanium which is relatively immune to oxidization and un-reactive with most chemicals. , however it is expensive and difficult to work with.

Next is Tungsten, however it is heavy

After that there are many alloys that use some of the metals above as ingredients such as stainless steel.

Finally most other metals are pretty much the same. Their friction coefficients and temperature resistances are similar, and their reactivity data all have some problems. As well some are brittle. The most important factor when using these is the lubricant.

Now I will rant on Inserts. Don't do it. There are many problems associated with inserts. Here are is a couple which in themselves are enough reason to not do it:

1. Dissimilar metals will cause galvanic corrosion
2. Different rates of expansion during heating and cooling will cause slippage