Pretty much any PowerBook will need its capacitors replaced and/or battery repair/replacement. On that note, my thoughts:
The PowerBook 150 is probably the most reliable 68k model because it was the cheapest and simplest model available. It was based mostly on the Duo 250 and in fact requires Duo RAM. Unfortunately you would need a special right-angle adapter in order to install a RAM module (these adapters were sold separately, of course). The use of IDE hard drives is definitely a plus and allows inexpensive installation of an IDE-to-flash adapter. The Casio PSTN display is 640x480 instead of the odd 640x400 resolution of most of the other 1xx series machines. It's usually a pretty consistent display, but it can fall victim to the surface bubbling that contemporary LCDs have exhibited. The battery is a standard 1xx series battery so they're easy to find and to re-cell if required. These models will also run with the Apple Low-Power AC Adapter that most other PowerBooks may refuse.
On the downside they don't have an FPU and require a special formatting program for the HD (not really a problem with later OSes but can be if you're running 7.1).
It's usually the active matrix 1xx displays that develop tunnel vision, whether grayscale or color, though the 170 gets it worst. I'd avoid those, though apparently some people have had luck running the machines for a few hours in a warm, dry place. It's theorized that moisture is causing the problem and the extended period of heat and dryness helps drive it out.
If you want color I'd suggest a 165c.
Duos are cool but as mentioned they have the worst problems with leaking capacitors. Plus the keyboards are total crap, especially in the early models, and the hinges can become weak over time.
5x0 series machines are very complex and have become very fragile. I'm fond of them but unless you find a gently-used model you'll likely have problems with broken plastics if you move it around too much.
The 1400 is probably the best non-G3 PPC model in terms of robustness, but the display housing is prone to cracking at the hinge mounts. Again, I'd probably avoid one for heavy portable use because these things are only becoming more fragile as they age. The 5300 is far too troublesome to be worth it. The 3400 and original PB G3 are cool but they are fickle in their old age. Leaking batteries (especially the PRAM batteries under the left speaker grille) kill them dead pretty quick. Plus they're really thick and heavy.
For G3s I'd recommend the PDQ. Once they got the problems sorted out, they were very quick and reliable machines. Plus they have CardBus support for using USB or FW cards and they have a full complement of legacy Mac ports, unlike the later models which first dropped ADB and then SCSI for on-board USB and then FW. Plus the Lombards and Pismos often had their own problems: keyboard failure, trackpad failure, hinge failure, broken plastics, LCD backlight failure, etc.
Honestly if you want a G3 portable, the clamshell iBooks are more robust and more reliable in the same way the PB 150 was: they were feature- and cost-reduced consumer models that used proven parts that were run well within their margins. Plus they don't have the internal PRAM battery to leak and/or prevent the machine from booting if it goes bad. The first of the white iBooks are also pretty good but of course the later 700+MHz models are prone to video system failure so I would avoid those unless you want to try to fix it.
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