I thought

RPG was okay but in my opinion not worth paying more than $10 for the cart of it.

RPG 2 likewise I think has been dull of what I've played of it. I haven't yet gotten to

RPG 3 or 4 yet - but I'll admit they seem to be nice to look at.
I like Rings of Power for

Genesis. Yes the game nobody has heard of, and ever fewer people have completed. The ending is awesome however and it is quite possibly the hugest game on a

Genesis cartridge. I like my RPG's to have some crust around the edges. Straight D&D RPG on IBM style... lovely. I didn't have the manual and only later did I find out that there was a map could be found in the game. Not knowing about the map was beneficial however, as it added a couple hundred hours to the playtime as I did a search on every single fricking square of the world. (Yes, I actually did) The game also had the best and most memorable music in my opinion.
Shadowrun on

Genesis although is likewise very intense and painfully difficult. So its another fave.
Tales of Phantasia on Super

is a fave - some of the best graphical effects on a cart I have seen. The bobble head characters in the GBA port make that version seem creepy to me however.
Earthbound, Chrono Trigger, and Dragon Warrior are obvious choices too. But everybody played those growing up I'm sure.
I'm just mentioning cartridge RPG's as CD RPG's is too easy.