I've become a big fan of the Ravensburger 3D landmark series and have bought most of them by now, and will probably keep on going.
Unlike the Big Ben and Eiffel Tower models, the Leaning Tower of Pisa has no hinged pieces, and is made up entirely of curved pieces. It's a lot easier to put together, maybe the easiest of the set, but it is also the one that works best as a traditional jigsaw - i.e. judging what goes where by the pattern rather than by the numbers on the back of the pieces.
I've read a couple of reviews of models in this series that complain about the numbers on the back of the pieces, as if that somehow makes it less of a puzzle. To my mind, the numbering actually increases the challenge as there are a number of pieces that are identically shaped and could fit in a few different places, so making sure it all fits together in the right order takes longer and is therefore a way of increasing the value for money.
It also has in its favour the best stand of the landmarks series - rather than tabs that are too small to hold the model securely on a cardboard base, you build it around a sturdy plastic base, and it's not going to fall over at the slightest knock, as the others are prone to.
The plastic pieces all fit together so well that once assembled it is absolutely smooth and works well as a traditional model for displaying.