The answer really depends on what you mean when you say “old” and “RPG”. And it depends on your system
If you mean really old and PC, i would suggest
Betrayal at Krondor
One of the few games in my life I just couldnt stop playing until Ive beaten it. Could get tricky to get it running today, I remember fighting with my 640k, if you know what I mean. DosBOX 4tw!
Diablo is still one of the best games out there (way better, not even compareable with its successors), but you probably have already played it.
Around the same time on consoles you just have to stick with squaresoft (almost). The SNES had so many excellent RPGs. Examples:
Secret of Mana
Chrono Trigger
Final Fantasy VI (or did you mean that by saying ff3?)
Zelda 3
Oh, just some note for ff6: the GBA version is way better. But to play it on a big screen, it would be necessary to download an emulator with the rom, but that would be illegal, so you should stick with the PSX version (it has an opening and ending cinematic, but it is buggy and has annoying loading times and stupid names for the spells and summons, so I would suggest watching the cinematics on utube and stick with the GBA version if you can).
After the DOS/SNES era, you should really play Final Fantasy VIII if you liked FF7. Its just a little bit more grown up, but the same quality.
And if other people are allowed to mention KOTOR 1+2 (and I really loved them too) or even Deus Ex : HR (which is mediocore and everything but old), I think im allowed to say Vampire: the Masquerade - Bloodlines. Just the best RPG Ive ever played, and I finished most of them. However you should get the latest inofficial patch, its buggy as hell in the vanilla version.
Would be nice to hear if you decided to play some games suggested in this thread.