Hydra is good for having MS effect, stam buff and the defensive CD works for some reason. 20% less dmg taken is pretty nice. Because of the extra stam and the CD, this is a popular pet because it lets you gear a little more glass cannon without losing any survivability. Or you can play like me and gear tanky, use a hydra pet and play mongo survival with 3200 health and 200 agi. Up to you.
If you're pure f2p, you might also enjoy the Hyena, which gives you the MS effect and a run speed buff which can help you kite vets who have run speed enchants. Also coolest looking pet imo.
Strider has a magic based slow that druids cant shift out of. Ugly as shit though, only use for sweaty games. Also kinda loses its usefulness if you have a team thats even remotely paying attention enough to CC druids. And its ugly. So ugly.
Pick either of those two to have with you and swap between them as game conditions dictate.