Regular tackles - open field "wrap" tackles - to be safe. Most boring tackles
Reach tackles- when you overpursue you can "reach" back and grab the ballcarrier. It mimics a "cutback" from the RB. Nice tool to use at LB especially to stop the run. The CPU does it ALL the time. CPU LB's oftentimes sit in a gap and reach at the RB when they finally attack the LOS. Also - use while engaged with a block! You can reach and grab a guy while being blocked (generally need the ability to do so). Makes ILB's better between the tackles for sure.
High Tackles - I believe would help tackle finesse RB's and Finesse MOVES in general. The hits are generally more violent than a regular tackle. I think it is the counter to finesse moves. Power moves or even just running can break through these tackels sometimes unless landed just right. ALSO from behind - horse collar tackles! FUN!
Low Tackles- Counter power moves - Stiff Arm, Branch Tackle, Battering Ram. Take out the legs while a guy is making a cut, or trying to run you over. Seems to work nicer with more frail tacklers to take out big guys. I believe you can finesse your way out of these a bit easier.
Big Hits - best part of playing defense
. Whenever you have the momentum advantage and the angle (think "downhill" as described above) you can land big shots fairly easy. They become intuitive after a reception or when a RB is stuck in a hole or making a cut to just POP them! Tough to land. Work great in a crowd (it's worth a try right?!). Low risk for big play situations especially against non-big play offensive players (FB's, slower RB's/TE/WR) it's great to at least try to blow the guy up and force a fumble.