Advice for upgrading RD
 

I'm currently riding a '74 Raleigh Grand Prix for my commuter, not too interested in restoring it or turning it into a show bike but I am interested in losing the terribly sticky & slow Simplex that was put on the bike when it was made. Not that I'm looking to upgrade to a Campy, but what are some quick & durable rear derailleurs that would be good for a daily commuter?

I would look at Suntour. A lot of folks at that time would have swapped out the original rear derailleur for a Suntour VGT or VGT-Luxe. They may be prone to developing pivot slop, but even so, they're so economical that you can pretty easily substitute another. And they shift as well as anything available at the time. Huret Duopar-eco might be another option.

+1 Pick up one of the many decent Suntour derailleurs out there. They are routinely on ebay for around $10 or so The VGT is a nice choice, as are the AR, the ARX, BL, Cyclone.

if your looking for new modern derailleur you best bet is a midlevel Shimano ATB. unless you want to troll through ebay for the afore mentioned Suntour ones.

PS I moved this out to the general C&V forum for you

YEAH! My Supercourse has a Suntour ARX on it that works quite well.

Thank you.

You'd be better served by changing out the freewheel for a shimano SIS compatible one. $20 with shipping if you shop around.

Then look into a new RD.

go for the ARX they are cheap and they are built like tanks. The grand prix is a heavy bike so a performance oriented derailleur won't help that much.

The old Deore mountain bike derailleurs also work great but you'll probably need a hanger.

I'm actually considering shooting for a Suntour Cyclone. From what I understand they are typically meant for 6-7 instead of 5, but I'm assuming that it won't be a big deal.