For hacks? Old phatties and some old slims, those are the only ones that support 3.55 and CFW. You can check the Slim models that are compatible with 3.55 here:
http://www.psdevwiki.com/ps3/SKU_Models
They can be fairly expensive, depending on what you're buying, and if you buy one above 3.55 you'll need to get a hardware flasher to downgrade them to 3.55 which can add $50+ depending on what you buy. You could also pay to have someone downgrade, which is usually cheaper than buying a flasher yourself, but if something goes wrong later or you accidentally update somehow you'll have to send it out again.
If you don't care about CFW, then realistically any one is fine. The newer slims are good, the Super Slims are ok, too.