Why do you want to know? I think we've probably all met people with intellectual disabilities and it's unfair to call them out. Met a guy who, as an adult, had suffered a head injury. While still retaining his personality it was heartbreaking to hear him talk about his past life, he knew what he'd lost and what he'd become. By no means the least intelligent but a good lesson to savour what we have. As for the most intelligent, there are many contenders - you get that when you work in IT - and I'd hate to have to cherry pick one as the "best of breed".
^This. There are people I've met in my life that weren't the most sophisticated people going, but they were intelligent in their own ways. Others I've known were neither street, people, or book smart, made the most horrendous decisions, and were happy to stay like that. I've been writing resumes for people lately, and I've come across some total brainiacs during my sojourn in resume writing. Skills, smarts, degrees, the whole shebang. The problem was they just lost faith in themselves and what they have to offer. Even if this forum were private, I wouldn't name them, b/c they're not here to defend themselves either way.
Probably won't blurt out names but just indicate my relation to them: most intelligent - A cousin of mine. Studied Microbiology just to get a scholarship and now working to get a master's degree on Business Management. A minimalist and a traveler. I consider him intelligent based on what he was able to achieve in both want and need. least intelligent - A former co-worker. Even if you were blunt to get your point across, he still wouldn't get it. At first, I thought he may have selective hearing but after seeing him fail all those online assessments the company required despite the preparation, let's just say he didn't last long. Six months later, I heard he applied for the same post again.