Here's my take on the three games:
GW1 - Favourite: The humour inherent in the shop-front signs and the landscaping in general buildings/signage etc. (very British - unfortunately that was partly lost in GW2 and is now non-existent in BfN) . A few of the maps were fun (can't remember the actual name of the map, but I remember it was about the zombies having to lock-down a sunflower lighthouse). Other maps were downright trivial.
GW1 - Least favourite: Balancing. The zombies had a clear advantage in fire-power and abilities, and only a handful of the plants were of any use (Sunflower who had the same task as the Scientist on the Zombie side was without merit. Others nearly as bad were Bandit Cactus and Commando Pea. Both were useless).
Some of the maps were pretty uninspiring particularly the one were one as a Zombie ended up trying to take over Dave's mansion.
GW2 - Favourite: After years of disastrous attempts at balancing it is practically perfect now, and by far the best in terms of balancing when compared to GW1 and in particular BfN - barring lag. So gameplay first and foremost. Some of the maps were great too, e.g. Zomburbia, Zombopolis.
Graphically speaking it is on par with GW1. In particular the wide range of game modes are (or rather were) superb in GW2. By far the best of all three Plant vs. Zombie games.
GW2 - Least favourite: It was and is present in all games, but lag was and is still irritating (eg. shows up particularly with Chomper when he burrows). In addition had you asked me two years ago, I would have said balancing would have been my main gripe.
BfN - Favourite: The open range maps. And that is it!
BfN - Least favourite: Game-play, balancing, abilities. Characters graphically a disaster when compared to GW1 and 2. Although recognizing that the game is targeted at mainly kids it is still excruciatingly childish. As I have pointed out in a previous message, I had expected a progression of gaming quality from GW1 all the way through to BfN, but sadly BfN lets the series down.
On an end-note, with the failings of BfN in mind I get the impression that the devs on BfN are new to the game and not the same as those who were on GW1 and 2.
