Maybe Aleksander for aoe def down and overall good nuking ( pve and pvp ) .  Or Gorgo who is more generaly useful in both pvp and PVE.  

Otherwise here's my take on a few of teh options : 

Vogoth potentially for doom tower ( or bosses in general ) is  a good Idea. 

Reliquary is pretty powerful but i'd suggest to see if you can make her work at 50 first.

PK won't be much value unless you pull an unkillable champ and even then she can work at 50. ( 60 to max damage of course ) 

Fayne could potentially be good for spider and later on CB

Caridnal is a bit outdated so only take him to 50 for faction wars if you're lacking options

That's great advice, cheers. I've cut my list down now

I'm not far away from another 60 so was thinking of Gorgorab next. Hadn't really considered Aleksander because I use a booked Spider in dungeons with high speed and accuracy. I don't have any legendary books and epics/rares are easier to book as I'm f2p. 

I think I'll wait to choose between Fayne or PK down the track depending on who I pull to get unkillable

I would take Vogoth as next lvl 60. That champ doesn't work at lvl 50. He is great at CB, but he needs to be lvl 60 for Giantslayer, otherwise he wouldn't benefit from his own leech debuff. Vogoth is further good as tank (for some magic affinity dungeon stages even off affinity), with lvl 50 he has not enough hp to do this job. Vogoth lvl 60 is much much better than Vogoth at lvl 50.

Gorgorab on the other hand can be a good arena tm and atk buffer with lvl 50. Of course Gorgorab is a candidate for lvl 60 as well, but not highest priority. You allready have Apo and Seeker as lvl 60 tm booster, Rector and Scyl as reviver. Gorgorab can stay lvl 50 for a while.

P.S.

Don't lvl up Paragon. If he is supposed to work as unkillable punching ball, he can do that better at lvl 40 with low hp.