Alpha numeric is a classic. Upper and lower case letters, with a few numbers thrown in for good measure. And optional punctuation and symbols if you tweak the parameters below.
Its absolutely fine (and recommended) to generate and store these in a password manager (although every password manager already does this for you), but they are a real pain to remember. Especially with more than 12 characters (the recommended minimum these days).
Use a Readable Passphrase, Dictionary Passphrase or Pronounceable Password if you need to memorise your password.