Assuming we're insulating on the "garage side", then I'd go with any of the above. Perhaps not finish with plasterboard in a cold garage Jonboy. No great problem with p/board, but it could soon deteriorate.
So many choices...........
battens with Kingspan/Celotex between them. Finish with cheap shuttering ply maybe.
2 x 2 (50mm x 50mm) timber with Kingspan/Celotex
4 x 2 (100 x 50) studwork with loft insulation between studs.
4 x 2 with 90mm Celotex between (getting really warm now)
I'm only repeating what the others have said really. You get the point. The only limitations are the width of the garage ....... and your pocket ;o)
Even ... test your trowel skills by slapping insulating blocks right up against the existing wall. (Don't bridge the dampcourse though)
My favourite is TLX Multifoil insulation (like a big silver duvet) but, since we're dealing with the "cold" side of the wall, TLX would need a bit more thought.
Nice, easy DIY job with any of these. Very effective too.