Which Power Armor is the most cost effective in terms of repair?

The level your power armor is upgraded to does not influence the repair cost (in my experience, wiki confirms), so if you haven't already done so, get your Armorer and Science built up so you can get to F or Mk. VI, as this will result in a marked increase in defensive rating without any additional repair costs.

Here are all the fully upgraded power armor sets (excluding uniques), including defensive values and repair costs:

Power Armor  | Physical | Energy | Radiation | Full Repair Cost
Raider II    | 620      | 250    | 1050      | 20 Steel
T-45f        | 1100     | 910    | 1050      | 27 Steel, 1 Circuitry
T-51f        | 1340     | 1090   | 1050      | 33 Steel, 1 Aluminum, 1 Circuitry
T-60f        | 1580     | 1245   | 1050      | 40 Steel, 8 Aluminum, 8 Plastic, 1 Circuitry
X-01 Mk. VI  | 1820     | 1390   | 1050      | 45 Steel, 16 Aluminum, 5 Copper, 2 Circuitry

The difference between T-45f and T-51f is negligible in terms of Aluminum (0 for T-45f and 1 for T-51f), but with a pretty substantial defensive boost. So, if you're using T-45 right now, I'd say go for T-51.

Last but not least, consider applying the Winterized Coating to boost your Energy Resistance (+20 with all pieces painted). It's not much but it doesn't add anything to the repair costs.