Have only really watched from BB5 onwards (and haven't seen anything after the first couple of weeks of BB12 as of yet).
Struggled somewhat to come up with 20, and they're listed in order of series rather than who I liked best.
Victor
Ahmed
Shell
Science
Derek
Makosi
Kemal
Eugene
Craig (not really as a person but good housemate)
Maxwell (as above, with added emphasis on him essentially being a piece of ****)
Glyn
Nikki
Shahbaz
Pete
Richard
Rex
Mario (BB9 - tosser but hilarious)
Angel
Sree (he made BB10 very entertaining in a trainwreck sort of way during the short time he was there imo)
Ben (only one I can really remember from BB11 even though I didn't mind it at the time)
Bonus most hated, in order of who I hated the most
Alex (BB9)
Grace
Sezer
Imogen
Sam (BB6)
Marco
Saskia
Lisa (BB10)
"It's a rong fing!"
Kris
Kenneth/Tom from BB10 - these guys were just creepy. Had to wiki BB10 to remember both of their names.
Jen
Anton (google Anton Murphy vs Shuffle-T to see him get destroyed in a rap battle - hilarious)
Dennis
Vanessa (BB6 - seemed like an alright person but boring as f... shouldn't have lasted as long as she did)