The only three that interest me are Multiple Man, Iceman, and Sunspot. Basically in that order too.
Sunspot’s powers would be a blast. Flight, solar absorption, strength, energy blasts. Kinda like a low level Superman.
Iceman’s powers would be fun too, but probably too destructive overall. Although being able to teleport through bodies of water could be a nice way to see the world. Ice clones would be handy too.
I’m choosing Madrox’s power since it can be used in many productive ways. Running low on money? Have some dupes work overtime or get another job completely. Need some housework done? A few dupes can knock out a complete remodel in no time. And you don’t even need to buy them beer or pizza. Want to learn some foreign languages? Send a couple dupes out in the world and within a few months you could easily be fluent in 5 to 6 more languages. Get that degree or doctorate? Just dupe it.