Pretty much anything in the appropriate length and weight rating is good, especially for free or already on hand. If starting from scratch I prefer Jeep CJ rear:I put leafs out of a Chevy Colorado under my trailer. Longer, wider leafs spread the flex over a longer area and seem to absorb bumps better too.
- common and inexpensive
- long enough for good performance but still fit under shorter trailers nicely
- the reproduction hangers are very sturdy, easy to obtain and weld up really well
- rear M20 u bolts are a reasonable fit on a 3500lb trailer axle
- CJ rear u bolt plates have shock mounts on them - a little less fab work to do
- huge selection of springs and shackles for stock and lifted on the market so you can play around with height and spring rate a bit
- CJs are major enthusiast vehicles so finding your bushings, bolts, spring friction pads, etc. is never a problem