I'm going to say anything new but I'd use a #Warcraft IV.
I started my adventure with these series at the time of Warcraft II — and it was a good game. I've tried "one" later, but it was too hard for me. If you haven't tried it yet, do try — and in the original version. I really want you to feel the pain of the original interface. Unfortunately, a game that comes with almost no player-assisting AI is not for me — one where you need to constantly watch everything, and manually tell every one of your units separately to defend themselves against the enemy.
StarCraft — of course it's a magnificent game. Unfortunately, it turned out too hard for me. Back when I last tried, I've managed to get to the Protoss campaign, and I think even the first mission was too hard for me. Perhaps if I played today, I would look around and figure some solution out, but how I am going to find time for games… By the way, do you happen to know if these modern remasters did anything about the difficulty level?
Warcraft III — all the best of SC ported to the rich Warcraft world. Great diversity of gameplay between the factions. This one I've completionated.
StarCraft II — a fair sequel to the first game, and this one was not too hard for me. That said, I see that there was a new DLC released since I've played, so perhaps it's time to return to it.
And World of Warcraft — I just think that's the worst that could have happened to the series. I just hate MMORPG, and I hate seeing all the effort directed to it, while all the good old RTS games are sidelined.
And yes, I'm old enough now to appreciate lower difficulty.