My favs are most Gottliebs from '51-'56. Most after that have the spin-roto feature which I really do not like. I like them because flipper games were relatively new, and although mostly symmetrical, you didn't see any playfield "Staples," or repetitious features, like return lanes, etc. Each game was much different from the last. A lot of variety.
This era also had several ways to earn replays, such as 4-5 different sequences + high score. A lot of them also had a multiple replay target or drop hole as well, such as mentioned on World Champ, Queen of Hearts, and many others. Most '70's games had only 1 sequence to try and complete to lite special, 2 on maybe a few more.
My favorite games would be Gtb. Niagara, Grand Slam, and Dragonette. I don't have enough time logged on Williams Woodies to render an opinion.