Ollie does seem lovely and genuinely compassionate. Him and Foxy are my faves but Foxy just edges it. I agree about Ant's ego getting in the way - a bit of a 'short man' thing going on there, I think, but still an interesting character.
Mark's back story was desperately sad and yes, he seemed a lovely guy. Also that poor guy a couple of years back who had listened to his wife being murdered by intruders whilst on the phone. Unimaginably horrific. So many of the recruits have been through terrible experiences but those two really stick in my mind.
For some reason I haven't enjoyed this series quite as much as the others (been watching since the start) but it's still one of my favourite TV shows.