These two words have a similar use to macanudo, that is they’re a positive way to refer to someone or something.
Someone copado is someone very cool and/or interesting. It can also express that someone is a nice person, very easy-going, with good vibes.
If you’re talking about an event or a thing, copado means the event/thing is also very cool, fun, exciting, etc.
Genial could be translated as brilliant, great, awesome, etc (I assume you get the vibe and I don’t need to keep writing synonyms ;)).
Genial can also be used to describe a person, an event or a thing. Of course this word is more intense than copado, so if you’re talking about an event and you say it was copado, it means that you’re glad you went; if the event was genial, it means we’d be sorry for not going. And while alguien copado is someone worth knowing, alguien genial is someone you have to meet. 🙂
About the mess created about the two meanings of the verb To Be in Spanish – that is, ser and estar – this is more or less how it goes (there are some exceptions, of course):
PEOPLE /STUFF= SER + COPADO/GENIAL
EVENT/STUFF= ESTAR + COPADO/GENIAL
Genial and copado, in the same way as macanudo, can also be used to agree with someone or something.
– Martín es re copado. Ayer nos estuvo contando de sus viajes por Asia. (Martín is really cool. Yesterday, he told us about his travels around Asia).
– El cumpleaños de María estuvo genial: había muchísima gente – todos muy copados – la música era buenísima, ¡y había canilla libre! Nos quedamos hasta las 8 de la mañana. (Maria’s birthday was awesome: there were a loooot of people – everyone was great – the music was fantastic and there was an open bar! We stayed there until 8 am).
– Todas las películas de Kubrick son geniales. (All Kubrick’s films are brilliant)
– ¿Vamos mañana a tomar algo? (Do you want to go tomorrow for drinks?)
– Dale, copado. (Great, let’s)