I haven't done as much Wolves den, but I would consider all of the teams we fought during the Ultros tournament top players. You can take a look at that thread for a list of people who entered.
In terms of Frontlines it's tough to single out a few players as it's more of a team thing, but the best group I've run against is the Sargatanas flames group, mad props to those guys. Getting wrecked by them was how my group significantly improved and I strongly believe that until you get crushed by a good team you're never going to be able to dramatically improve.
In terms of single best person on our data center (which i'm judging based off of seeing them solo queue) I'd say Gryzz Odyss followed very closely by Momoko/Popoto. Gryzz used to do a lot of solo queuing and I can count quite a few games where he queued alone and was the sole reason we either lost or had a closer game than expected. When we saw the choco mask it really got the adrenaline pumping.
To be a top player in Frontlines takes both skill, but also leadership. Knowing who to focus, when to push, when to run, and when to hit mid often has more impact on the outcome than individual fighting skill.
Gotta give credit to where credit is due right? Here is my list of top players I've ever had the pleasure of facing on Primal DC ever since 2.1 on their prime. I've always learned something when facing them. Forgive me ahead of time if I dont list all of you, I have terrible memory
WAR - Knights Wrath, Lisa Summers, Tsubasa Neko
PLD - Mona Carmona, Master Dan
MNK - Saucy Racer, Hinata Hyuga, Koltik Morrel, Byooki Braus
DRG - General Esdeath, Mona Carmona, Freya Gemini
NIN - Curty Ballistar, Bahamut Zero
BRD - Epistane Havoc, Lord Garuda, Copy Cat, Lisa Summers
BLM - Kisada Exis, Jack Nicholson, Tsubasa Neko
SMN - Tea Pan, Pho Killa, Rece Ballistar
WHM - Halie Winters, Virtuouso Chyea, Rece Ballistar, C'eve Arthu
SCH - Ayerro Valrya, Rum Raisen, Freya Gemini, Bannajuice Hades