Well there are still a few Pokemon from generation 1 that are still powerful forces in today's OU metagame. However I will name the ones that I personally see as the most prominent ones:
One of the most famous Pokemon and very well loved Pokemon from generation 1 still functions as a reigning monarch in today's OU metagame. With incredible stats and an excellent movepool I believe that Dragonite is still the most powerful generation 1 Pokemon that you will ever run into in the OU tier. Seeing as it got graced with Extremespeed in generation IV, Dragonite is one of the most feared priority users and with it's respectable bulk and breakthrough ability in Multiscale it is almost guaranteed that it will live at least one powerful hit
Spooky in generation 1 and still spooky in generation V without getting any major updates to it's old set. How does this prankster of ghost do it? High Speed, High special attack, beautiful movepool. Gengar has kept it's title of top-OU Pokemon throughout 5 generations mainly because it is too powerful to be in the lower tiers and it still hits like a truck. With it's immunity to one of the most powerful offensive types, fighting Gengar can make the lives of fighting type Pokemon that are stuck into one fighting move because the are choiced so miserable. Also being one of the fastest and most offensive non-legendary ghost type in the game it laughs in the face of common rapid spinners as this move is rendered useless once Gengar decides to switch in and will therefore keep the entry hazards stably on the field. With all these wonderful abilities it is only normal it still finds it home in the OU tier
"Gyarados, oh Gyarados, why does thou maketh our lives so bitter in our own Tier?" Gyarados has a draconic movepool and stat distribution but unlike them he bares a neutrality against ice type and dragon type attacks making him a powerful con dragon that utterly eviscerate many Pokemon in the tier as he has been doing for V generations. However in the 5th generation Game Freak decided to give it another boost in offensive and they did. They gave it the ability Moxie that immediately made him one of the most offensive Water type Pokemon in the entire metagame. However the ability it has had for V generations, Intimidate still enabled it to be an incredibly bulky water type in the OU metagame. With this it is one of those generation 1 Pokemon that have always resided in the OU tier and will most likely forevermore.
Starmie is that lovely star that you see here and there. It's role in the OU tier is vital for many teams and thus it is still in the OU tier. It is one of the only spinners that can actually get passed gengar as it outpaces it and it has a STAB move that will obliterate Gengar before it can even pronounce the P in prank. With an excellent movepool and incredible typing, Starmie is still an OU force in generation V
Last but not least we have Venusaur. One of the 3 original starters. With the release of geneartion V, Game Freak decided to let the light fall on it again and with this gave it the Hidden Ability Chlorophyll which has made it one of the deadliest Sun sweepers in today's OU metagame as once it's special attack has reached 2X, you might as well quite as it will outpace the entire unboosted tier and kill everything that comes it way with it's STAB moves. With this Venusaur is one of the original Pokemon that has made it in the Ou tier