It depends very much on what lighting, substrate, equipment, hard or soft water etc you have. If you are running a low light tank with just gravel then Java ferns, java moss, anubias, cryptocorynes, hornwort, egeria densa, many of the Hygrophilas are great fast growers. Limnophila sessiliflora grows easily and looks good I think. Ludwigia repens is pretty easy and goes red if you have enough light. Amazon swords and vallis pretty easy too. There are lots more but those are among the plants I have found the easiest. As I said depends on the conditions in your tank and whether you are going to use ferts or not.
