Ah, “best”…
I use Vex IQ with grades 2-5 mixed together, kids starting at various times, so everyone at different levels. I start everyone with the virtual bot (“mimic”) Hour of Code that Robot Mesh offers, then have them build that bot, then go through the (nearly identical) “Real Bot” Hour of Code at Robot Mesh. I then move them to Robot Mesh Studio, mix in giving them next-step programming assignments, modifying their bots, building other builds from https://www.vexrobotics.com/vexiq/resources/robot-builds, and day-dreaming up whatever they can invent. I show them VEX IQ Robot Showcase - VEX Forum especially builds by @Valeria93 and @Vexatron. Whatever they build I ask them to program. I use the controllers sparingly at first, almost as reward or fatigue breaker; later we do more with them.
I have a classroom package from RM and use the local desktop application (via Parallels on my Mac at home; Windows at school) to look at the students’ saved programs and add comments for them to consider.
I have also taken the RobotC based instructor training presented by Carnegie Mellon, and I show them that environment after RM. To me, the RobotC “Graphical” environment seems intended as an intro to the RobotC text environment. See this topic: Robot C Graphical (Multi-Task) . The third post, by @Jason63, says it clearly. My totally non-expert opinion is that the linear, limited approach of Graphical RobotC might teach tighter coding skills. Lots of other discussion around about this, like Why Start with C++?.
I then show them Modkit, and I routinely write a simple program in all three environments, screenshot and Photoshop them to print and laminate for the students to inspect, and have them demonstrate all three on bots. I am using Modkit more lately, the various videos online are useful, see Would anyone be willing to share a sample code for VEX IQ Highriser and/or Ringmaster? - #10.
I have all three applications on the laptops, and eventually they pick a favorite, though I encourage them to try all three.
There are some fun errors in the various curriculum… I do encourage you to try the examples on real bots yourself before trying to help the students.
Always good to have a plan B, software and firmware can need updating, the web-based RM environment requires good internet, a 2nd grader might walk in and “get it” all in two days and be bored without inspirational challenges while a 5th grader is “getting” nothing and wants help on everything.
At this point I put in 2 full days of prep for each 3 hour class, making sure each bot and laptop will actually download and run, Windows is updated, drivers are reloaded if necessary, charging batteries, etc., and staying ahead of the students’ learning.
So, “best”? All three are useful in different ways. Try them yourself, you’ll need to be somewhat fluent to guide students…
Best job I’ve ever had.