Big question that one.
How long are you likely to be in it? Long time? Then a pee zip or pee valve are great.
A couple of hours, man up and hold it
Rear zip, front zip or over the head bib style. Bib means pure self donning. Rear is easiest to get in and out of but needs some one to help, front zip I find a pain to get in and out and can be prone to leak before a rear one, they do however mean you can pop the bottom for venting or weeing.
Breathable or not? Breathable allows any sweat to vent off, it will not stop you sweating. Base layers can help in this by moving the sweat. In time the pores will block and fail anyhow so is it worth the extra cost? Some will say yes, other may not.
Neoprene or membrane? Neo has it's own warmth properties, membrane allows you to pick your warmth via base layers. Neo can be less flexible but stronger and resistant to abrasion.
Seals? Neo can be more comfortable, latex has better waterproofing. Self changing or glued on. Self change is simple but they can be lumpy and get in the way.
Boots - membrane socks, built in boots, latex socks, latex ankle seals? Ankles seals are ok when tied in with neoprene boots but can leave cold spots. Latex socks allow any sort of boot to go over, cheap to repair but prone to breaking / splitting. Membrane socks are great but usually mean you need to go up a size. Built in boots are common and seem to work well. I have three suits, one of each.
Do you need a full suit? Dry trousers and dry cag can be more comfortable. Dry trousers and coat works well too.
It usually comes down to price - what can you afford and what is available locally. Needs to fit well to be comfy and safe to wear.
Decathalon have some good prices on Gul suits at present, these are also available on the net. Two of mine are Polar Bears and are MTm - they are great but not cheap. My lightweight is a Typhoon and really is cheap and cheerful but great for a short hop and easy to throw on.