What is the best way to connect multiple Arduinos?
I think you should start with using Wifi modules and you use an esp-01 too instead of NodeMCU. Once you are familiar with esp-01's, you will find them as good as NodeMCU. Esp-01's only lack multiple pins when compared to NodeMCU. 

And for communication between arduino and esp-01, use serial ports- 0 and 1. 

Here is example code-
For Master Arduino 

int temp;
int hum;

void setup(){
 Serial.begin(115200);
 delay(2000);
}
void loop() {

 temp = random(0,100);
 hum = random(100,200); 
 Serial.print(temp);
 Serial.println('t');
 delay(2000);
 Serial.print(hum);
 Serial.println('h');
 delay(2000);
 Serial.flush();
}


For Slave Esp or any other slave arduino -

String inString = "";    // string to hold input

float temp = 0,hum = 0;

boolean newtemp=false, newhum=false;

void setup() {
  Serial.begin(115200);
  Serial.println("ESP 8266 - 01 Module is ready to receive codes from Master Arduino");
 }

void loop() {
  // Read serial input:
  while (Serial.available() &gt; 0) {
    int inChar = Serial.read();

    if (inChar != '\n') { 

      if (inChar == 't')
      newtemp = true;

      if (inChar == 'h')
      newhum = true;

      inString += (char)inChar;
    }
    else {
      Serial.print("Input string: ");
      Serial.println(inString);
      if ( newtemp == true ) {
      temp = inString.toFloat();
      Serial.print("Received Temperature:");
      Serial.println(temp);   
      newtemp = false;
      }
      if ( newhum == true ) {
      hum = inString.toFloat();
      Serial.print("Received Humidity:");
      Serial.println(hum);
      newhum = false;   
      }  
      inString = "";
    }
  }
}


Pin connections - 

Master Arduino TX -- Slave board RX