SeaNile31- Welcome to Autopia!
Ya know, oddly enough I was sorta stumped for an answer to your seemingly oh-so-simple question! Gee, what *do* I consider must-have products? Not as easy as it might've seemed :think:
Of all the swell stuff already mentioned, the only one I use on a regular basis is FK425. While it's a *great* QD, I dunno if it's a "must-have" if you like what you're using now.
I almost always counsel people to concentrate on the wash, and for that I *do* have some must-have suggestions, but that's just based on how *I* wash cars. I couldn't do without a foamgun, Boar's Hair Brush ("BHB"), and soft mitts. From there, it's all about technique as the best stuff in the world won't help avoid wash-induced marring if you use it incorrectly.
I do like Waffle Weave MicroFiber ("WWMF") towels for drying.
Beyond the wash/dry it's a question of how far you want to take things; it's a virtual certainty that your paint is marred (scratches/swirls/etc.) and that gets into the topic of polishing, which is one mighty involved subject.
Last Step Products ("LSP"s), i.e., waxes and sealants, are pretty much a personal preference topic, but there are some that are generally better/worse for most situations.
Most interior maintenance can be handled OK without getting too fancy about it, especially if you keep things from getting too nasty in the first place.
SO...I guess my question back to you is: what do you want to change/improve?