Wetsuits can be a tricky bit of dive equipment to buy. Realistically though if you are a travelling diver in an ideal world you would own at least three different thickness of wetsuits to cater for global water temperatures.
But here in the real world and due to us all having limited budgets, our usual recommendation is that if you are only going to buy one wetsuit then make it a 5mm wetsuit. A 5mm wetsuit is the most versatile and will cover the widest range of diving and snorkelling temperatures and sea conditions. Diving in the Red Sea, Mediterranean summer diving and perhaps even mid Summer in the UK with a 5mm wetsuit and a shortie overvest or hooded chickenvest could do. Whereas a 3mm wetsuit will usually cover you for Caribbean, Red Sea in summer, Indonesia, Maldives or Thailand. Then for the chillier dives either a 7mm wetsuit or semi-dry wetsuit for the UK.