New Filter, What Would You Recommend?

[Blondielovesfish]

Hi all.
I currently have a 21L tank that I will be turning into a Nano soon for some Guppies and I was wondering what filter I should have. 
I currently have an AquaOne Maxi 101F internal filter but I'm not happy with it as the suction cups don't really work (they stick on for about 1-2 hours then the filter is just hanging limp in the water until I fix it) and also I'm missing parts (I bought it about 3 years ago and never noticed they were missing until this year. )
So I would like to get a new one.
What would be the recommended flow?
What size tank should it be able to cope with?
Any you recommend? 
I would like to stay with internal filters as the top of my tank was made wrong so HOB filters don't work.
 
Thank you for any help offered.

 

[Miller2009]

I have a 25 litre tank currently not used due to upgrading but when did I used a fluval u1 internal filter worked great for me and my guppies seemed fine with it too.