If buying new I would look at JBL, PPI, Alpine, and Phoenix gold for amps although PG is probably out of a budget price range. For headunit, Pioneer, Alpine, Clarion, etc really depends on what you are looking for. For subs I would look at jbl, lanzar opti, tc epic, ssa dcon, alpine type r, with many many more all depending on budget and wants sound wise. For speakers I would look at jbl, alpine, etc but these are very listener specific such as I hate harsh tweeters so I never liked infinity but their speakers are nice to many people.
Used you can get some better products all over the board but you take the chance on issues or abuse. There are many good sellers on the forums you just have to pick and choose and lay down a baseline of what you really like from a listening standpoint to start separating brands.