My favorite would be our Vivarium Electronics TH100. It was like $12 at an expo so i imagine online they'd be dirt cheap. 1x3 inch nice display. Long probe lead. Its our longest lasting temp/humidity gauge. Exo Terra 2/1 are good too, my issue with them is over time is, where the wire runs into the probe, something, usually animals bump, move, or climb on it. The connection comes loose, and part of it stops reading... so hide it well lol. Theres a variety of options. Theres also ones from department or hardware stores
No wires, battery power. Issue there is you have to hide a 1or2x4 sensor or work it into your tank so it doesnt stick out like a sore thumb. Have one that does temp/humidity inside and outside(the sensor) to keep an eye on my room temp as well. A steady room enviroment helps.

So theres a wide variety of options out there. Kinda comes down to preferance.