Expansion Bellows - Ideal Position

There is no ideal position, any position is a compromise of multiple factors. If you hope to benefit from an informed opinion you should provide more information.

What kind of information is required?

A detailed drawing of the heat exchanger(s) and all associated piping, hangers supports and braces along with the materials, operating temperatures, pressures and flow rates. Add a ranking of your needs and this information would provide enough for many of us to take a wild guess about the solution you seek.

I don't think you are referring the expansion bellow at the shell, right?

This expansion bellow on exchanger shell is to accommodate the (relative) expansion differences between shell and the tube bundle.

If you are referring the expansion bellow at the pipe lines, more so with tube side line than the shell side pipes.

Normally, vertical exchangers' (shell and tube type with fixed tube-sheet) 'Support Lugs' are designed such a way that they are as closer/nearer to top as possible. Such that the maximum expansion of the whole exchanger (if any) will be on one side (downside) and such expansion is fully accommodated by a bellow. If this be the case the suitable place should be bottom tube side connection.

You really should deal with a local engineering firm specializing in thermal expansion and pipes/ducts. There are many factors to be taken into consideration - not a minor task that it seems to be.

The following assembly of vertical shell-and-tube heat exchanger, BEM type, which furnished with expansion joint is commonly and practically used.