The quick answer: whatever fits your budget and feels best.
Everybody's body is different, every manufacturer's suit is different.
I would recommend this plan:
Decide your budget (Prices range from entry level to oh, wow.)
Decide your style of suit:
Do you want full-length and fully sleeved (long john style) or sleeveless or a shorty?
Decide the features that are most important to you.
Don't forget to consider the lifespan of the suit as it relates to your "career" as a triathlete. A good entry level suit can get you pretty far.
So as not to be overwhelmed with choices, I picked three companies to look at: Orca (because of the hot guy in the ad, honestly), Blue Seventy (well-known) and Body Glove. Since I knew squat about any of them, it was as good a method of choosing as any. I also looked around on the internet for opinions about suits and these names came up quite often (very positively so, most of the time).
So I had my three companies.
THEN
I set a budget and prioritized what I was looking for.
Since I am short (5'0) and female (XX) I prioritized my needs like this:
ABSOLUTE PRIORITY:
Fits my budget with the most bang for the buck.
I am an entry-level triathlete. Or almost athlete
I need something that will work and last for the next several seasons becasue it will take time for me to "outgrow" (if ever) the suit.
MUST HAVE:
full (long-john style) suit, since I won't be doing a lot of warm-water races.
a women's specific fit
warantee or guarantee against defects, etc.
Doesn't require complicated care or maintenance.
easy exit, since I'm exciteable and would be a complete
spaz coming out of the water and trying to get the darn
thing off while heading for T1
GOOD TO HAVE:
extended warantee
panels in shoulders to allow for better mobility, since I have some shoulder problems and i don't want to waste effort trying to "power" against the resistance of the suit, esp. in the shoulders
BONUS IF I GET IT, OK IF I DON'T
it would be nice if it had some purple (or pink) in it =)
DON'T REALLY NEED IT
Extra "speed" features, right now, i just want to finish.
Any sort of other "Elite" features that drive up the price point.
I then found a suit (or suits) from each company and made spreadsheets to compare features side-by-side.
I am in the boonies, so I had to order via the internet. I found companies with generous return policies, ordered a couple of suits and tried them on, ultimately selecting one for myself.
All the suits made me look like a little black bratwurst sausage, but I found one I liked (and it made me look like a FAST bratwurst, too!)
However, if you can, go to local shops, do so. You can find places to rent suits, too. This is a good way to try out different kinds.
The base for "size" on a wetsuit is weight and you go from there. The suit should fit snugly (but not strangle-ly) around your neck, the last thing you want is something interfering with your breathing! T should be tight around ankles and wrists. You want to make sure you have enough mobility in the shoulders, so as not to affect your swimming stroke.
Most people (including myself) who don't have the best form in swimming, get a lot of help from the extra buoyancy provided by a suit. Because of this, "pool" triathlons and tri's where the water is generally over 79 degrees don't allow suits. Always check with your race organizer!
Hope this helps you pick!