BTD1 was not a very hard game. Sure, you didn't have Road Spikes, but the maximum bloon rank was white, which means you could beat the game with un-upgraded Dart Monkeys (which I did). BTD2 was a bit trickier...I did manage to beat it on easy, but it was tough. BTD3 was really the transition between the simplistic feel of the earlier BTD installments and the more open feel of the later installments. I only managed to get as far as round 58, though. One big thing they introduced in BTD3 was Free Play mode -- up until then, you just played to a very limited number of rounds. It really felt as if a big restriction was being removed.
BTD4 was the one which really started the evolution of the series. A ranking system, new game modes besides the normal ones, unlockable towers and upgrades...it was the first really popular game in the series. It was also the first to have a spin-off, which was BTD4 Expansion. While the MOAB was a big jump forward in BTD3, where really wasn't a very good counter to it since the popping power was still pretty limited. However, with the introduction of the Sun God and MOAB Mauler upgrades, all of that changed. You suddenly started seeing lots of MOABs coming out at once, and yet you were able to pop them. BFBs were actually easier in BTD4 than MOABs were in BTD3.
And then, of course, came BTD5 and all of its expansions. I think there's a reason that so many different version of BTD5 have been made: it's because there's no point in making a BTD6 with all of the different versions of BTD5. Pretty much everything is covered: Cooperative mode within BTD5; PVP with BTD Battles; even a real-time game with Bloons Monkey City. Of course, the original BTD5 had some major changes. Not only did you have to unlock towers by ranking up, you also had to use the towers to unlock their upgrades. You have Monkey Money added in, with which you can buy Special Agents -- a new feature of gameplay by themselves -- and Specialty Buildings. Additionally, Daily Missions and Special Missions were added for extra tough challenges. BTD5 and its spin-offs were -- and are -- just about anything you could ever ask for in a tower defense game.