Excluding series with only one or two games, because they're just too easy, this is a tough one. I think I have only two viable candidates:
- Emperor: Rise of the Middle Kingdom - The series is long dead, and I've only played the last two (Zeus and Emperor), but I have a hard time believing the earlier games are better unless you have nostalgia for them. Between Zeus and Emperor, Emperor has always felt way better to me. It's just more refined in probably every way.
- Ratchet & Clank: Rift Apart - It's not perfect, but it also doesn't have any major weaknesses. I suspect the second game in the series is the only one I could find to possibly be able to compete with Rift Apart, but I don't remember it well enough anymore. I vaguely recall it having even fewer weaknesses, but undoubtedly the controls would feel dated at this point, so I'm confident enough to say Rift Apart is better.
I was also going to mention Uncharted, but Uncharted 4 isn't the latest game - The Lost Legacy is, and that one, while good, simply isn't even a contender for the best game of the series. Besides, I think I personally prefer Uncharted 2 despite Uncharted 4 having superior gameplay, so it's not clear Uncharted would belong here anyway.
Also, obviously it wouldn't probably be too hard to come up with series that I haven't played (or haven't played enough) where the latest game is the best. The Witcher is probably a prime candidate, but I cannot vouch for it personally just yet. Baldur's Gate might be one too, but again, I don't have enough experience with it to tell.
