I think Gundam is the series I've been the most invested into and still finding new ways of appreciating and learning things in about every series. I think each Gundam series and timeline is good up to around 2015 ending with Reconguista in G, which I feel is the last truly special series that only Freaks like me appreciate, but was divisive though found a following.
Each Entry has it's own unique strengths and weaknesses, in a way I think isn't something that's approachable like any other series. Because I think it's the most popular and important anime series that internationally I don't think anyone has really "got", I guess that's the territory of when you're a series that changed Japan and has been spanning for almost 50 years, that a lot of things are lost in people, Imagine if Star Trek was only popular in the states and the rest of the world had to play catch up.
Anyways, I think the best/favorite entries really depend and change but for me the ones I think of are like 00, X, Victory and Zeta in terms of shows. For movies, It's Char's Counterattack, Awakening of the Trail Blazer, and Seed Freedom. For OVAs, you can't go wrong with any of the UC ones from 0080 to Igloo, because they are that fucking good, Though I do lean towards 08th Ms Team pretty often. I do think there is plenty of good to be within each of the shows up to a point, Even Shows like Age and Destiny which people consider "Terrible", I think are have their flaws but still have plenty of positives that do shine. But the best ones, I think are some of the best anime I've seen in either TV or Movie form and ones I do think about often.