Hi Matt,
I've used several different apps with various titles and to date anyway,
DashMeterPro is the Android app of choice for me. I did have to purchase it 3 times (iRacing, AC/pCars and rf1/rf2) but I still feel it's worth the price of admission.
Before DMP I tried iHud and although they started off well, support is nearly non-existent now and their add on package apps never did work for me so I would not recommend them.
I have also tried a few free one's and usually they either display too little info., have too much lag or simply aren't "pretty" enough for me.
I thought I'd mention them here anyway, just in case others were curious...
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SimRacingTools free app (AC, Race07/GTR/GTL)
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SinSpeed a very basic free app (AC, iRacing, rF2 etc.)
There are also SLi/app units that combine telemetry info. with Sli (rev lights/indicator) for even more versatility.
SimInstrumentsDash does a good job of keeping their software up to date with new titles but these will run you close to $200.
Hope that's helpful.
Bailey