What should I sincerely call a nice stranger who helps me

Sir, if it's a guy or Ma'am, if it's a woman. It's considered cute if you call a child sir or ma'am. If they are a young adult they might not like being called sir or ma'am because it makes them feel old, this is more common with women. Then you can call them Miss. If you know their first name you can call them Miss \[First name\] or Mr \[First name\]. It is showing respect but not too much.

"Many thanks, kind stranger" is quite nice - if somewhat quixotic. In person, "Sir" is always nice - but can be problematic on the internet, due to the risk of misgendering someone; therefore - to be honest - I often avoid the issue entirely by saying "thanks" without an epithet. When I do need to address someone here, I usually refer to them as "OP", or by their username, or occasionally as the singular "they". I avoid him/her, and other gendered terms. A simple "thanks" is never amiss. For Brits, "Cheers!" - and if you know they're British, calling them "mate" is rarely deemed offensive. Personally, I call everyone "duck", but that's a regional affectation, and often causes confusion. In Somerset, they call everyone "my lover", which is even more problematic.

“Miss” if it’s a woman. 

Ma'am or sir in the southern US. Other dialects will vary. Miss/sir, probably

I personally wouldn't call anyone anything in this situation - for being grateful to a random stranger on the internet I would just say "Hey thank you so much that's really helpful" or something similar. You don't always need a term.

buddy isn’t offensive but in certain contexts it can be sarcastic

Seeing you're asking about online interactions, there's no reason to use any particular name or term at all. Just reply to the person directly with a nice thank you. That's rare enough on Reddit! Trying to find the right term to use is a bit of a minefield, because you're talking to an international audience and things can vary from place to place. Best to steer clear and keep it simple.

A stranger who helps you is a ‘good Samaritan’. This phrase originated in Christianity. You could say this to someone: “you are a Good Samaritan.’ but they might not understand, and you can’t use it to address someone: “Hey, Good Samaritan.” You could just say: “Thank you, I really appreciate it.“

"friend". In usage: To them: "Thankyou so much my friend" To addressing another while they are still there: "I am so thankful to my friend here for helping me out" Talking about them later: "I met a new friend today who helped me with..."

The adjective would, I believe be something akin to a stranger's being cordial , or mayhap convivial, or simply helpful

Where are you? Huge divide in UK vs US vs Australia for this. Australia and the UK are similar but I think the US is quite different. ***For the UK*** **Option 1 - Nothing** "Thanks so much" "Appreciated" "Cheers" "Thanks" "Nice one, thanks" **Option 2 - Mate** "Nice one mate" "Cheers mate" "Thanks mate" "Appreciate that mate, thank you" In the UK "mate" is generally a unisex term. NB it almost (almost) always comes at the end of a sentence in this context. **Addressing women - If you don't look like a massive sex pest, you can likely also get away with "love" when addressing a woman.** "Cheers love" "Nice one love, thank you" "Appreciate that love, mind how you go" There are other variants (for both sexes) that vary by region (duck, pet, darlin', pal, buddy, butt, boyo) but avoid these unless you know what you are doing. "Mate" is almost universally ok. If you say "Sir", "Ma'am", "Miss", "good samaritan", "good fellow" or anything even remotely formal, you will get looked at like some kind of weird eccentric. Don't use these.

Friend. Buddy, bud, bud or pal, can be sarcastic, but as an honorific “friend hasn’t fallen into that. “Thanks friend”

“You, sir, are a scholar and a gentleman.”

I say "friend" sometimes, but I think this is pretty uncommon.

I don’t think a term is needed, just a genuine thanks, and you have shown you already know how to express that!

I just say thank you friend or something to that effect