The "biggest" YouTuber I've met is probably Jim Chapman. I met him at a show press night in London back in 2014 and asked him if he'd mind being in my vlog for a few seconds saying hi as I was vlogging the evening. He was really lovely about it and I still have that video on my channel.
I then bumped into another British YouTuber (who I won't name, but he has over 100,000 subs) on my way to work last year. I was with a colleague and decided to say hi to him. Although he seemed nice enough, I found him to be a little bit arrogant. For instance, as soon as I told him I watch him on YouTube and love his channel, he immediately started going on about his latest videos and how good they are and that I should defo watch them. (I wasn't a big fan of this YouTuber, I was just subscribed to him as I enjoyed some of his videos). Also, he asked me if I wanted a photo! I didn't, of course. I'd just wanted to say hi and I found him a bit self-important to assume I want a photo. To avoid awkwardness though, I said yes. My colleague took a couple of photos of us on my phone and as soon as we left, my colleague (who'd never heard or seen this YouTuber before) told me she thought he was arrogant and not likeable.
Then just a few months ago, I bumped into a group of smaller YouTubers (50,000 - 100,000 subs) in a restaurant in London. Their table was on the way out, so I decided to quickly go up to them and say I enjoy their videos before leaving, simply because I genuinely thought they'd appreciate it. This was the most awkward encounter I've eve think! The conversation went something like "Hey I watch you guys on YouTube, you're all great"... One of the guys just stared at me, a girl (also a YouTuber) sat next to him kind of smiled and nodded, but literally... it was mainly silence. I then just turned around and left as I didn't know what else to do. I found that incredibly rude and have since unsubscribed to them all. I realise it may have been unexpected for them, or they may just be awkward people (they do come across awkward on their channels too), but this was just ridiculous.
I only have about 2,300 subs and have been approached by viewers randomly about 30 or so times. Even though I'm always taken aback when people recognise me, I try my best to be nice and friendly and make sure the people knows I appreciate them, even if I feel awkward. It's not hard to be polite.