The MicroB web browser is probably the first that any Maemo user comes across. It's the default browser, based on Gecko, and is a continuation of the line of browsers on earlier Nokia Internet tablets. Of the three browsers, it's the most hildonized, feeling most at home on the platform. It uses the same status-bar slide-in to call-up customization options and Maemo notifications as the rest of the platform. It has some stability issues, however, and is internally the least consistent. Areas of the browser that are less common, such as extensions and plug-ins, did not get the hildonzied appearance resulting in needing to use an old-fashioned scroll bar.
The Midori web browser is the second option. It's based on the same rending engine as Mobile Safari and the Android's web browser. It renders about the same, and has built-in support for user-agent spoofing so that sites treat it just like if you were using an Apple iPhone mobile device instead of the higher-resolution N900. There are some rendering differences, which I haven't quite figured out the cause of given that they do both use the same underlying engine. Midori also has shown some odd behaviours, like forgetting about touch-scrolling at one point until it was restarted.
Finally, Mozilla Fennec, the code name for the mobile version of Mozilla FireFox. While MicroB supports limited ad-ons and extensions, Fennec is a mostly-compatible version of the desktop FireFox browser and supports many of the same ad-ons. The UI work that went into customizing Fennec for the N900 is impressive, and it does feel very natural. With that said, there are a few things I miss from MicroB, such as the "swirling" magnification. Fennec has only a double-tap that changes from zoomed-in to zoomed-out (somewhat reminiscent of Opera Mini's "map" feature), but does not allow one to continue to zoom in like MicroB does. On the other hand, some may find that double-tapping as a toggle makes zooming through a page quicker than MicroB which continues to zoom in on each double tap.
The summary report is that I'm using a combination of the three. With many sites wanting to treat everything as an iPhone, I use Midori in iPhone-spoofing mode to see what sites consider their superior mobile presentation, and am trying Fennec for most normal browsing right now, mostly because of its support for tabs which MicroB lacks (but Midori does have).