Here's how it goes for me-
BN1- the first, actually quite an original kind of game when it came out. I give it kudos for introducing a new style of gameplay. Solid gameplay but it was short and easy for me. Also was outdone by most of the sequels. Generic story and progression (not that this has ever hurt a Megaman game though). Nice plot twist at the end, to break up the monotony. (IMO) To be honest, I found this one closer to classic Megaman games, in terms of story and progression. Fight robot masters, progress. That's how it has always been. The later games became more dynamic in this respect.
BN2- what an improvement! Takes everything from the first game and somehow puts it on steroids. I don't know exactly how to describe it, but the game was longer, harder, and even more hardcore. I think a couple of characters actually swear, lol! Kind of a darker story this time. Cool villains IMO. The best new addition are the forms Megaman can evolve into. Style changes are fun.
BN3- Even MORE of an improvement over BN2. I love the navi customization system, adds some amazing depth to it. More style changes, better story, and even further improved combat. Felt better balanced at times. BN3 i'd call the best in the series. Some people like BN2 more though.
BN4- Um, what happened? The change in graphics is hit or miss, but the things that tick me off are the throwaway badly paced story and unbalanced gameplay. You don't become involved in the real plot until near the very end of the game, and it's just like "oh yeah, we forgot, go save the world. K thanks bye". I didn't like anything much about this game. Combat could be fun, darkchips were better used in the next game, and it just feels like a huge step backwards. I'd consider this the worst in the series, even worse than BN1 or BN5.
BN5- This game had huge potential and is not without its merits. The story is really good, and the gameplay is half great. Too bad Capcom had to throw the liberation missions into the mixed. Boring and tedious strategy RPG elements does not a good Megaman game make. And they're a core gameplay element. If they hadn't included them, the game would have been 10x better. As it is, it brings the game down to second worse, just above BN4. Sad I am.
BN6- Nice finale indeed. Brings back what was great about the 2nd and 3rd games and brings the evolved combat of the older games. Nice mix of everything, and a good way to say bye to the series and send it off in style. I'd consider this the third best. The only flaw is that there's not much new about it.
The Starforce series I consider hit and miss. It's like Capcom got two strikes and then a home run. Starforce 1 and 2 were rather like a poor man's battle network. But SF 3 stands proud on its own. I can play it and see that they actually tried in it. It's quite unique, despite sharing some core mechanics of the previous 2 games. There are just a select few new things that make the game so much funner. Primarily, being able to pulse into the wave road whenever you want. And the noise system reminds me of the old style changes of BN 2 and 3 multiplied with the cybeast system of BN 6. Again, I assume this to be the last Starforce game, and I found it a nice way to say goodbye to the series.
^my personal opinions of course, not yours.