ESX Performance Tool ESXTOP
http://www.vfrank.org/esxtop/
http://www.yellow-bricks.com/esxtop/

Disk Performance
Device Latency not to exceed 25ms | Kernel & Queue Latency not to exceed 2ms | Total latency not to exceed 27ms
Counters to use:
Queue command latency
Physical device read latency
Command latency
Kernel command latency
Kernel read latency
Bus resets
Queue read latency
Physical device write latency

Network Performance
Look for dropped packets, and usage in KBPS, and check for errors on the physical switch port.
Counters to use:
Data receive rate
Data transmit rate
Receive packets dropped
Transmit packets dropped
Usage

Performance Best Practice for VMware 6.0
https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/techpaper/vmware-perfbest-practices-vsphere6-0-white-paper.pdf

Veeam One Monitoring
https://www.veeam.com/virtual-server-management-one-free.html

Multipathing in vSphere using EQL MEM
https://vmpete.com/2012/09/20/multipathing-in-vsphere-with-the-dell-equallogic-multipathing-extension-module-mem/

Best Practices for Implementing VMware vSphere in a Dell PS Series Storage Environment
http://en.community.dell.com/techcenter/extras/m/white_papers/20434601

MEM Performance for VMware Essentials Customers
https://www.nexusos.co.uk/content/news/blog-articles/tech/mem-performance-for-vmware-essentials-customers.ashx

VM Health Monitor
https://www.manageengine.com/products/vm-health-monitor/free-vmware-health-monitor-index.html