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If you aren't willing to do any work to make the page valid, don't even worry about it -- the DOCTYPE is the least of your worries.
If you're interested in writing valid, portable code, go with a Strict DOCTYPE, and validate using the W3C tool. Fall back to Transitional only if you can identify the features of your webpage that force you to do so and can describe the effects it will have on rendering in IE, Opera, Safari and Firefox.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">If you try the above the page and the page layout becomes adversely affected, use this instead:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">The second doctype (without the link to the DTD) will ensure that the browser remains in "quirks mode".
Once you have your doctype in place, validate against it and start working through the errors one by one from the top. Don't be disheartened if the validator says there are hundreds of errors - problems can cascade so fixing earlier errors can often solve subsequent ones.
If you use the second doctype listed above, once you have fixed the errors in the page, it is a good idea to revert to the first one (which triggers standards-compliance mode in modern browsers) and fix any display bugs. After that, you can start simplifying your markup and introducing more CSS to lighten up the page and prepare for a move to validation against a strict doctype.
Good luck!
