Haven't played many series and not all installments of a series but here goes..
My all-time favourite game is GK 3, and it's also my favourite game in the series, GK 2 running a very, very close second. Broken Sword has already been mentioned; the first game in the series is, to me, the best of the four, with the Smoking Mirror and the Angel of Death on 2nd and 3rd place. Sleeping Dragon I found to be the weakest game in the series for various reasons (controls, stealth/action sequences, crates).
As for Monkey Island, I've only completed the original MI and Curse of MI; I liked the latter better but couldn't really put my finger on it why this is the case. Still have to complete LeChuck's Revenge and am still hesitating whether to by Escape from MI or not. The controls are a major put-off (I just don't like keyboard/gamepad controls), and the graphics look weird to me. Usually I don't care much for graphics even though I do enjoy good artwork; but in this case, they just look strange. Don't know if I could look at them for hours while playing the game
As for other series, I've played Dracula: Resurrection and liked it a lot. The Last Sanctuary still sits on my shelf, waiting to be played. From what I've seen so far, it's a lot gloomier than its predecessor, and it has action sequences (kind of) which makes it more intense than Resurrection.