I would say that most filters will do just fine with 20 liters of water without the recommended maintenance.
If there was one I would consider it would be Sawyer Squeeze. The Squeeze is a hollow fiber filter which can be forced (squeezed). It is affordable, reliable and simple to use. The water is available immediately; you can even drink as you filter. I find it easier to use than a pump-style filter. You may also use it as part of a gravity system.
The Squeeze requires back-flushing using a syringe. In my experience, this is rarely required if the water is relatively clean. You may choose to bring the syringe with you although I personally would not. Another option is to mail it to yourself for long hikes (i.e. bounce box). With clear water it repeatedly worked just fine for weeks-long stretches. When it requires back-flushing you will notice reduced flow rates.
The filter is remarkably simple and does not have many moving parts. I did have problems with the O-Ring becoming misshapen, which resulted in leaks and un-usability. The problem corrected itself after a certain time. You should not tighten the bottle too hard as it may cause issues (now I know).
I also owned a MSR HyperFlow but had it failed on me in the field. I would not recommend it as it is more error-prone and more costly than the Squeeze. Ceramic filters are also reliable and field maintainable. However, I personally had bad luck with those as the cartridge broke down on a trip.
For non-filters, you could use a drops such as Aquamira. I would recommend transferring the contents of the bottles into a secondary container as the corners of the bottles tend to crack and leak. I've had it happen to me twice and I have met other hikers with the same problem. I believe Aquamira is aware of the problem but I do not know if they have corrected it. You could also keep them in the original plastic box which offers some extra protection for those sad cornered bottles.
SteriPens are great but unreliable for multi-week trips and costly (battery usage).