best exp servers are the most populated ones because you'll always find people to exp with most likely best way to figure that out is through ffxiah.com > database heres a link though.
http://www.ffxiah.com/data.php
you could also make a mule on every server do /sea all during times of average but anyway here are the results
Most populated : Odin 56,477 characters
2nd Most populated : Bahamut 50,125 characters
3rd Most populated: Asura 47,343
all other servers are 40,000 or less on average.
People might go on a spree saying odin is too populated and say gtfo etc but on average NA time has about 2100 people on average weekday JPish time 2500 people on weekend NA time 2500-3000 people average weekend jpish time 3000-4000 peopleish. Highest i've seen was 4500 and its been a long while since i've seen that many its normally on NA saturday near midnight time because its prime time for both NA and JP and some EU.
If you consider the fact that this is how almost every server was populated about 4 years ago it really isnt that many people. It gets annoying when there isn't anything good going on in LS (which is majority of a lot of peoples FF time) and you can't find anyone to exp with or do random shout crap (lolshoutgroups). when you did a /sea all 10-50 on my old server you only saw 200-300 people if you were lucky which made hell trying to form any exp pt or get invited to any. Normally you'd wait 3 hours and get a shitty exp pt with a 4 level gap and it'll disband because some newb had to go. this was main reason I moved to Odin and now i'm finding I'm enjoying my FF time a lot more because it actually has people on the server. oh and if you try to shout for cop/toau missions or something you'll actually find people here too i guess.
P.S. if your trying to get any rare / ex items from a nm though it'd probably be best to do it before server transfer though because you'll most likely have 0 or maybe 1 person if your unlucky on your server. I normally encounter 0-2 people except for that freak time at moldavite where this guy had an alliance of 18 people camping it.