From the 3, I definitely prefer Dota 2.
HotS is a fantastic MOBA for people who don't want to pour hundreds or thousands of hours into it, and the interactive maps are a lot of fun, great for beginners, good charm and characters and matches are finished relatively quickly. Unfortunately, if someone wants a super complex and deep experience, the game is limited and eventually you're going to look into something else.
LoL to me is in the middle ground of being easy to pick up and what you can do with your skill level, a lot of champions have fun personalities with abundant skillshots, and I understand not everyone liked the skill tree mastery (If it still has that, it's been a while since I've played) but I thought it added fun versatility to each character. I don't like how champions are deadlocked to their roles and map placement and trying out new things can get you banned. Stuff like that means that your meta never changes. But I do like that it has different maps with different modes.
With DOTA 2, I'm biased because I've played so much of the first DOTA that it's the gameplay that I love the most. I love the art style that Valve has created, and the fact that every single hero is available from the start, meaning skill level is the only difference between you and each person. I love how most heroes can be played completely differently and map placement can vary like crazy. It's encouraged, not looked down upon, to experiment. Thankfully with stuff like custom games, it's insane how many different types of games you can play. I also love that while it has the highest skill ceiling, there are some heroes that are clearly meant to test your APM and accuracy even if you're not a pro. There are things like a lack of early flash and the fact that denying is super important which I really like, but I completely understand why that would be a turn off for others.
You can't go wrong with any of those, but one thing is that with a group of friends, all MOBAS are elevated in terms of fun unless your friends are tryhard assholes.