0.030" MIG wire (solid wire, not flux core) works ok. I use .035" all the time for sheet metal since that's what I keep loaded for general all purpose welding. .023" would be better but it birdnests really bad unless you have a spool gun.
For oxy-acetylene, 0.050" or 1/16" filler rod will suffice.
Stick-weld: forget it
TIG: if you have a TIG welder, you wouldn't be asking.