Here's a list of all the blocos. As a general rule of thumb, avoid anything bigger than 20,000 people especially the massive ones in Ipanema, Copacabana and Centro (attracts a lot of pickpocketing, sometimes drunken fights etc).

Rola Preguiçosa - Feb 20 @8pm
Imagino? Agora Amassa - Feb 22 @10am
Suvaco do Cristo - Feb 23 @10am
Cordao do Boitata - Feb 23 [MENTION=5397]12pm[/MENTION]
Empolga as 9 - Feb 23 @4pm
Samba de Santa Clara - Feb 24 @2pm

Personally, I find the blocos in Santa Teresa to be the most fun, although it's a bit of a pain to get to (Uber works fine, do not wander around the neighbourhood and get lost after the bloco finishes).

Bloco das Carmelitas - Feb 20 @3pm
Ceu na Terra - Feb 22 [MENTION=623495]8am[/MENTION]

Another tip: stick to buying cans of beers/water bottles at the blocos (no risk of being spiked). Avoid anything like caipirinhas, and those home-made smirnoff ice lollies.