I'm going to clarify my answer. I think there are 2 different 'strong' types of strains.
1. Usually Indica, knock you into a lazy state of mind and lock you to the couch, sleepy
2. Usually Sativa or hybrids that produce a psychedelic experience, paranoid, anxiety, mess with your mind, tripping type of experience
I would say #2 constitutes 'stronger'. There are some good landraces that claim to do this, Durban Poison, Thai, etc. Also Hazes and Jack Herer strains like Cindy 99.
I compiled a list a while ago from information I got online on trippy strains. I can't vouch for it's accuracy but I'd be willing to bet its more correct than not so:
Aceh – Trippy, strong landrace strain from Indonesia
Acid – A racy acid-trip like inducing diesel cross
Alaskan Ice – Reputed to be one of the most powerful, heady strains around
Alaskan Thunderfuck – A creeper strain that packs a potent head rush
Amnesia Haze – So strong it makes smokers forget
Apollo 11 – A super potent Jack Herer psychedelic strain
Apollo 13 - A super potent Jack Herer psychedelic strain
Arcata Trainwreck – One of the most well known psychedelic strains around
Big Sur Holy Weed – A powerful, psychedelic strain from California
Belladonna – Well known for inducing paranoia
Black Domina – A strong indica with paranoia inducing properties
Black Magic African – Reputed to be one of the raciest strains around
Casey Jones – A racy diesel cross
Cinderella 99 – Well known as one of the most racy strains around
Clockwork Orange – An Alaskan Ice hybrid with devastating mental properties
Columbian Gold – An old school strain with a reputation for inducing paranoia and racy thoughts
Destroyer – With Thai and South American ancestry, this strain is reputed to ‘destroy’ the mind
Dr Grinspoon – A unique sativa with a racy, strong high
Durban Poison – An African strain with a reputation for inducing paranoia and racy thoughts
East Coast Sour Diesel – Well known to induce paranoia in unsuspecting users
Exodus Psychosis – A racy, paranoia inducing skunk variety
Ghost Train Haze - A super potent psychedelic strain
Golden Tiger – A very psychedelic Asian hybrid
Hao Bac Silverleaf – Also known as Vietnamese Mindfuck, this potent strain is well known to be trippy
Jack Herer – Well known as one of the most racy strains around
Jack’s Cleaner – A super potent Jack Herer psychedelic strain
Jack The Ripper - A super potent Jack Herer psychedelic strain
Juicy Fruit/Fruity Juice – Reputed to induce paranoia
LA Confidential – A strong indica with paranoia inducing properties
LSD – Reputed to induce an acid-trip like experience
Malawi Gold – An African strain with a serious reputation for inducing paranoia
Mullumbimby Madness X Thai 78 – Very strong sativa with mind numbing potency
Nanan Bouclou – A landrace reputed to have serious paranoia inducing properties
Nebula – Reputed to induce paranoia and racy thoughts
Nigerian – An old school strain with a reputation for inducing paranoia and racy thoughts
Punto Rojo – An old school strain with a reputation for being trippy and inducing paranoia and racy thoughts
Romulan – Reputed to be so powerful it can ‘dent your head’ like a Romulan
Schrom – A very strong Columbian Romulan mix that is known to cause paranoia and hallucinations in some smokers
Snowcap – A strong indica with paranoia inducing properties
Strawberry Cough – Psychedelic strain with extremely long lasting high
Super Silver Haze – One of the most notorious paranoia inducing strains around
Taskenti – A trippy strain from Uzbekistan
Texada Timewarp – A strain reputed to warp time, this heavy hitter popular outdoor strain in BC packs real potency.
Thai – Many thai strains are well known for their paranoia inducing effects
Trinity – A rare clone only strain reputed to induce paranoia in some users
Vietnamese Black – Extremely trippy, well known for inducing paranoia, like most Vietnamese strains
Vietnamese Laughing Grass – A strain comprised of Dalat, a trippy, psychedelic landrace from Vietnam
Vortex - A super potent Jack Herer psychedelic strain
William’s Wonder – A strong indica with paranoia inducing properties
Willie Nelson – A racy sativa cross that induces paranoia in many users
White Widow - A strong indica with paranoia inducing properties
