CommanderX may be the most overlooked and underrated caster in the community. He has a good voice for casting and has a lot of experience. IMO he was tailor made for being a color commentator but could main cast just as well. CommanderX would regularly say why or how some push or tactic didn't work in a quick and accurate way, while not lingering too long at the expense of the current action. Not many casters can do this well and they just keep talking about a push that failed 90 seconds ago while new plays are currently happening and not being casted accurately.
Arx was a caster I wish did more. He had such a great casting voice and would stay on top of the action non stop. He was an animal during team fights and transitions. He rarely had good casting partners from the casts I have seen and would have benefited a lot from a solid color commentator.
Admirable was a good caster when he wasn't saying stupid things about EU compared to NA. Unfortunately, in lots of casts, this would just drown on for the entire map and would get annoying and repetitive. A few fun loving jabs I can take but sometimes it never ended. Still, he did a fine job of relaying information and almost always had a solid grasp of the team situation, a skill most casters don't develop.
cbear was probably the best caster who could talk smartly about team fights. Casters today and at i55, i58, etc would not be able to process all the action in a team fight and simplify it by focusing on who "is down" during a team fight. They basically just report deaths in the killfeed and the fight itself is not being casted. Being able to still focus on the team fight and provide that "feel" for listeners is a difficult casting skill to have and cbear seemed to do it regularly.
gecks has/had the finest casting voice of them all IMO. He would almost sound like a sports caster at times and it was enjoyable to watch those matches. gecks would have a good grasp of the situation with ubers / positioning and relay that to listeners really well. The gecks/sideshow casting pair was as good as it could get IMO.
Sideshow is probably the caster with the highest awareness of changes. He would almost always spot an offclass or a switch to kritz or some other situational nuance before it had an effect on the match. I think this allowed a lot of the great plays in TF2 LAN's to be viewed real time because the Cameraman was informed of who to watch on the fly by the caster. Sideshow could sound overanxious/excited at times but that was usually just his passion for the game that took over. Can't say I have a problem with it myself.
There was an old EU caster called Pledge and he was the most passionate sounding caster I have ever heard for TF2. He always sounded so into whatever he was casting and made listening to just about any match exciting. He didn't care who was playing, it was all trying to professionally cast for him. It was sorta like listening to a soccer match and once the action started, his voice would get stronger and he did a fine job relaying the action. I don't think he was paired with many strong EU casters back then and it's a shame because he would have been the ideal "color commentator" to support a main caster.
I personally enjoy Bren as a caster as well. He has a clear, strong voice and does a fine job staying up with the action. He did a fantastic job at the last i-series. He would remain calm even during messy team fights with an overload of information.
Lots of casters panic in many situations and have serious trouble relaying that information quickly and accurately before it happens. I wouldn't want to name anyone or put them on the spot and talk bad about them here. However, there were many who simply didn't do a good job. That said, I appreciate the effort and the willingness to cast matches. Those guys and many others put in free time for the rest of us to enjoy TF2 and they do it for no pay.