IMO Ranger is the weakest, worst-designed class in the mid-late game. Your spell list isn't very good, and otherwise you're just a nerfed Fighter. If you just want to be an archer, I would recommend playing Fighter. If you really want to be a Ranger for RP reasons or whatever, I'd still recommend multi-classing a few levels into Fighter.
Warlock is my personal favorite class. You're not the best caster but Eldritch Blast builds can be very strong and you have a lot flexibility to choose the tools you want.
Multi-classing serves two purposes:
1. Several classes (including Ranger) don't actually get much in the way of useful class features after level 8 or so, so dipping into another class lets you pile on more useful stuff. For example, a single level in Fighter gives you proficiency in all armour and weapons plus a Fighting Style, a second Fighter level gives you Action Surge, and the third Fighter level gives you a subclass. All of these are potentially very strong with the right build.
2. Some class combos let you do pretty gamebreaking things. For example, pure Paladins never get spell slots above 3rd level, but if you multiclass with Bard or Sorceror you get more and better spell slots so you can Smite like crazy.
