What is the best bridge for a stratocaster?

For what?

Its no use having a hardtail if you want to emulate eddie van halen.

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No no and no

A profound statement.

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You dig?

Just something that will help the guitar stay in tune

Well for tuning stability you'll get no better than a floyd rose or kahler.

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No no and no

If your guitar isn't staying in tune it most likely Isn't your bridge,it could be your tuners slipping or the strings binding in the nut(the strings could be too big for the slots)or binding on the bridge.If it's string binding that can be remedied by putting graphite or some other string lube in the nut slots and on the bridge.If the slots are too narrow you could either use a lighter gauge string or have a qualified tech file the slots.Another possibility is too many winds around the string post,2 or 3 winds are plenty,also you should stretch the strings as you tune after putting on new ones.

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'65 Strat,65 Mustang,65 Jaguar,4 more Strats,3 vintage Vox guitars,5 Vox amps,'69 Bassman with a '68 2-15 Bassman cab,36 guitars total-15asst'd amps total,2 vintage '60s Hammond organs & a myriad of effects-with a few rare vintage ones.

What would be a bridge similar to a kahler, but cheaper?

If this is really a tuning stability question, then consider these fixes before changing out the bridge ...
1) Lubricate with graphite the nut, trees, and saddles
2) Go to a 'no float' configuration
3) Tighten down on the bridge by adding a spring, or screwing in the claw screws some.
4) Possibly after #1-3 ... Have the nut sanded out a bit if it seems the nut is binding the string

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Laughing out loud with fear and hope, I have a desperate plan ...